I would not recommend skipping the sheathing, which could be either generic osb or ZIP. Using the (taped) ZIP you can eliminate the Tyvek, or you could prime and tape the OSB. Either method, done well, forms the "air barrier" on the exterior. A layer of foam on the exterior is generally a good idea for the thermal break (which makes your interior insulation more efficient) plus it will give you additional R value. As far as the type and thickness of foam, there are water vapor issues which vary by climate/location and which others can address. Adding 1x3 strapping over the foam (or sheathing if you don't use foam) will give you a "rainscreen" which can prolong the life of your siding.

I'd use some exterior foam and I'd look into airtight drywall plus taped foam - you don't want any air circulating from either side.

Shear panels set flush with the exterior stud edge, or cut in bracing can be used to stiffen up the wall against racking forces, but I dunno...  a full skin of OSB detailed as an air-barrier feels more secure to me.

Priming 2" either side of the seams with a quality acrylic latex  as a bonding surface for housewrap tape works fine as an air-barrier, but doesn't take the place of a weather-resistant barrier.  Some XPS foam is now rated as a weather-resistant barrier if the horizontal seams are taped with housewrap tape using a Z-flashing methodology.   In 1-story applications using 9' XPS rather than 8 eliminates the horizontal seams, and may serve that function with less detailing (especially if 2 layers are used, with lapped seams.)

Whether the sheathing & framing are detailed as the primary air barrier or not, it's still worth using air-tight drywall methods on the interior.

ICF is the easiest way to insulate the foundation. Using a poly capillary break between the footing and the walls minimizes the wicking of ground moisture up into the foundation wall, and a metal sill gasket at the studwall that extends beyond the foam in both directions minimizes termite risk as well as moisture migration from concrete into the wood.

In Zone 4 there's a long-term net-present-value argument for 2" of EPS under the basement slab as well. See Table 0.2 on p.10 of this document.  Note, the recommended R-values are whole-wall, not center-cavity or clear wall.  To hit the BSC-recommended R25 for Zone 4 with an advanced framing 2x6 sprayed/blown-cellulose approach takes 2" of exterior XPS (2 layers of 1", seams lapped) or ~1.5" of iso, or 2.5" of EPS.  Any of the above provide sufficient exterior R to be able to use foil-faced goods on the exterior, and  standard latex paint on the interior in your climate.  But unfaced rigid foam with a vented rainscreen gap between the siding & foam will enhance drying toward the exterior, making the assembly somewhat more resilient.

Thanks for the help. 

Just wanted to clarify on the placement of housewrap - is it between the foam and sheathing or on the exterior of the foam?

Vented attics are the norm around here.  What's the best idea for attic/roof design and insulation?  I haven't planned on any cathedral ceilings, so I'm assuming I'd go vented but I'm open to options.

As long as there aren't ducts up there, then a vented attic is less expensive to insulate, less heated sq feet and serves as a large vented rainscreen gap (a good thing).

Install the housewrap against the sheathing and under the foam.
You can get a better air seal with that placement.

What jonr said about vented vs. unvented- if design the ducts air handlers out of the attic an you'll have a much cheaper place to insulate to a high-R.

Designing the ceiling & trusses to be able to handle the depth & weight of 1.5-1.8lb/cubic-foot cellulose is cheaper and more effective than going with a lighter-density and more expesive fiberglass blowing wool.  Batts basically suck-there will always be gaps & voids, and lose R at big temperature differences.  Blown fiber at moderate to high density works best.  But prior to insulation attic floor/conditioned ceiling has to be meticulously air-sealed at every plumbing & electrical penetration & seam.  In some instance it's worth using a full coverage of 1" closed cell foam, which also becomes a ~1perm interior-side vapor retarder, but usually it can be spot-sealed with foams & caulks for less money, and (when necessary) the vapor permeability can be adjusted with paint.  Design it such that you get at least 3" of fiber insulation over the joists or truss-chords as a thermal break to get the best performance out of it.  If you can, blower-door test & rectify any leakage prior to blowing the attic insulation, since it's your last-best & cheapest time to get a good air seal. Make any access or inspection hatches as small as you can, and use rigid foam to insulate those.  If  it's an operable hatch (as opposed to a break-out) take care to use a quality weatherstripping.

In Zone 4 (St. Louis down to about the Arkansas border) doing attics as mostly-cellulose there's a long-term financial argument for ~R60 (thermal bridging of joists included), which is well in excess of the  R38 code min.  That takes about 20" of cellulose (settled-depth- start out with 24") which will run 2.5-3lbs per square foot of static load.  Count on  using 5/8" ceiling gypsum or a layer of OSB for carrying the load without bowing over time, and provide for either taller studs on the upper floor or an "energy heel" truss to be able to keep it at full depth all the way out to the exterior wall sheathing.   Be sure the joists or truss chords are rated for the static load as well- it may require fatter chord elements or narrower spacing to get there, but there is always a way if you design it in ahead of time.

Avoid using recessed lighting on the upper floors too- that ends up creating thin spots in the insulation and heat-driven air leaks.

The housewrap over/under the foam issue depends on how the windows are installed & flashed- it's not a simple either/or.  Both the foam and the housewrap should be detailed as air-barriers, independently of where they are in the stackup.

Wasn't there something in the new eStar code that the foam should be 1.5 inches over 2X6 24 OC walls, to prevent condensation?

-Rosalinda

The amount of foam required to eliminate condensation on either the sheathing or the interior gypsum is highly dependent on local climate (and center-cavity R of the fiber), the perm rating of the foam, and whether there's a rainscreen gap between the siding & foam.  While 1.5" of exterior XPS is probably good for 99% of the homes in the lower 48 if the wall assembly rainscreened, that much isn't necessary to be protective in mixed heating/cooling climates.

In St. Louis the binned average outdoor temp for the month of January is about 29F. As long as the average temp of the interior face of the sheathing stays above the dew point of the interior air, condensing hours in the coldest weeks are still fewer than drying hours.  The typcial rule of thumb that works is to assume 70F air, with 35% RH air, which has a dew point of ~40F.  As long as a bit more than 25% of the center-cavity R is in foam & siding, there's no appreciable moisture accumulation in the sheathing.  Center-cavity R on a 5.5" cavity with cellulose or fiberglass is going to be R18-R20, so 1.5" of XPS (R7.5) would be absolutely safe, but even 1" would still work with a rainscreen gap.  1.5" of foil-faced iso (~R9) would also be safe, but only 1" (R6) is on the edge for St. Louis, since rainscreened or not, there's ZERO drying toward the exterior through foil facers.

There's far more to it than that though- this is just the crudest of dew-point calculations.  The wicking of cellulose increases the resiliance since condensation moisture is safely pulled away from the structural wood by the cellulose and stored, to dry seasonally etc.   With modest interior vapor retardency (3-5 perms of standard latex paint) and reasonable air-tightness on the interior 1" of exterior iso would be enough to be protective even in places much cooler than St. Louis.

Also, if a flash'n'fill technique is used as the air-seal, the 1" of closed cell foam puts a ~ 1-1.5 perm vapor retarder on the interior face of the sheathing air which limits the rate of moisture diffusion from the cavity into the wood, yet allows seasonal drying to the interior, making the thickness & vapor retardency of exterior foam pretty much irrelevant in US climate zones 6 or lower. The wintertime condensing surface in a flash'n'fill would be the foam, which won't wick toward the sheathing.  It's good to have at least SOME foam or vapor retarder on the exterior of the sheathing though, particularly on masonry-clad buildings in hot-humid climates to keep the high vapor-drives of sun-on-rainsoaked brick from raising the humidity of the sheathing, since  the flash-foamed interior slows down it's drying toward the air-conditioned interior.