"Radiant" and "cost effective" don't always belong in the same sentence. But "radiant" and "comfort do, so it's a choice. The high cost of a retrofit radiant floor system will never be offset in lower marginal operating cost.
Yes you can zone with an older boiler. There will be more thermal mass in the system with a radiant floor of equal output to your baseboards (unless they are cast-iron baseboards?), but you may still have to add thermal buffering if you micro-zone the hell out of it, or if the boiler is already oversized for the existing radiation. There are lots of things to get right to make it work well even with a new modulating-condensing boiler.
Don't sweat the hardwoods issue unless your heat loads are so high that you have to run 180F water even on an average winter day.
If the ducts go in the crawl space (and you air seal & insulate the crawl space if it isn't already) a heat pump solution would be cheaper up front, and also have very low operating cost. While standard split-system heat pumps could work here, most have fairly poor low outdoor temperature performance, and require resistance heating strips to make up the difference on cold weeks. But there are effective (and often cheaper) modulating mini-split solutions, both ducted and ductless that might work.
Putting ducts in the attic can be easier and cheaper to install, but it will add at LEAST a ton to the peak cooling loads, and is far less efficient in both heating & cooling modes than having the ducts completely inside of conditioned space.
A typical ranch house with R30 or better in the attic will have a cooling load on the order of a ton per 1000', sometimes more if there is a lot of west-facing window, often less if there is at least partial shade and limited west-facing glass. Mini-splits put out ~15-18000 BTU/hr per ton of cooling rating, and many are rated for -5F or colder operation these days, whereas most standard (or even modulating) split-systems have capacity issue even at +5F. While that's below the +11F 99% outside design temp for Salt Lake, it's pretty close to the design temp for Provo. With a heat pump it would be good to spec something that at least has -5F or lower as an operating temperture range, even if you're specifying it for it's capacity at +11F. (For outside design temps, see: http://www.energystar.gov/ia/partners/bldrs_lenders_raters/downloads/Outdoor_Design_Conditions_508.pdf ) With MODULATING heat pumps (split or mini-split) the comfort levels are pretty high, with much steadier room temps and much less noise & wind-chill compared to on/off or two-stage hot air solutions, but they don't deliver the same barefoot comfort as radiant floors.
A modulating heat pump would be a somewhat higher cost much higher comfort than a gas furnace with a cooling coil in the air handler solution, but nowhere near the cost of a multi-zoned radiant floor maximum-comfort solution.
Before picking ANY heating solution you first have to get a handle on the actual heat load. With the nameplate efficiency of the existing boiler and some mid to late winter gas bills the whole-house heat load can be estimated fairly accurately, but it won't necessarily tell you how that breaks down by room or zone. If the room-to-room temps with the existing baseboard are pretty even, it's possible to infer the room loads as a fraction of the total by the lengths of baseboard serving those zones as a fraction of the total. That's not quite as good as a room-by-room Manual-J calculation, but it's a lot better than most HVAC contractors in my neighborhood seem to muster. (YMMV, and I hope it does!) If some rooms are warmer colder than other you can still use the proportional baseboard lengths to figure out which direction the new radiation (or other) needs to be skewed.
So, what boiler model/make, or what are the input BTU & DOE output BTU?
Got a ZIP code (for weather data precision), and late-December through mid-March gas bills (with the exact meter reading dates and fuel use numbers)??
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