OK, so we're not actually using SIPs, but that's about as close as I can describe. 
We're doing a modified Post and Beam, something like this:
http://www.firstdaycottage.com/pictures/building.html

Ie, we're doing a continuous type of panelized system (T&G interior, foam board insulation, OSB/ply for wracking strength, board and batten siding)

here's my question:
We're building in NW Kansas.  We can get pretty cold in the winter, but we also get blisteringly hot in the summer.  Our humidity tends to be pretty low.  So, I'm trying to decide how best to approach both a vapor barrier as well as a radiant barrier.

At the moment, heat source will be a centralized wood and pellet stove with baseboard electric as backup and zone heating.
A/C will be a couple of window units.
The house is a basic 24x40, gable roofed, 1.5 story over a walkout basement (with garage in the west half of the basement).

Any suggestions are welcomed!

Spend a bunch of time at www.buildingscience.com, especially researching the articles about designing for cold climates. You'll probably realize that you don't want to use a vapor barrier, you will want to seal every possible air leak, and insulation on the underside of the roof is better than radiant barrier. Radiant barrier has definite benefits, but it's not the be all to end all. If you do go that way look at OSB roof sheathing that has an aluminized bottom side. Other than in the attic, radiant barrier is mostly a waste of money. Take a good hard look at insulation systems as discussed on that site. You'll find the pink panther does a better job of hyping than saving energy.

Radiant barriers have the greatest value when using fiberglass insulation (which is semi-transparent to infra-red). It will not have a measurable effect when used in conjunction with a SIP.

In the roof structure radiang barrier would have value when applied to the underside of the if your insulating it with rock wool or fiberglass (batting or blown) on the floor.

But it's much better to go with an unvented, insulated roof structure (and forget about shingle warrantees, which are a joke in the first place- you'll lose at most 10% of life of the product.) See:

http://www.buildingscience.com/documents/digests/bsd-149-unvented-roof-assemblies-for-all-climates

It's easier to air seal it perfectly that way, and it guarantees that the pressure boundary of the building envelope is the same as the thermal boundary (less confusing to people working on it, who may otherwise punch holes in the attic floor and not seal them causing infiltration problems, etc.)

SIPs are de-facto class-I vapor retarders. But in dry climates like KS addtional vapor retarders cause more problems than they solve- don't add them to your non-SIP structures.

But AIR-barriers are still useful, as is foam-sealing all plumbing/wiring penetrations, taping & mastic-sealin seams to the SIPs, etc. Orders of magnitude more moisture get air-transported than vapor-permeated through wall/roof/floor structures anyway- stop the air, you've stopped the condensation problem (in all but the very coldest climates of the lower-48) but absence of vapor barrier allows a diffusion escape path for any moisture that does manage to find it's way inside the structures.