Wheel and Tire suggestions
With your current wheels, you should be able to go down to 700x30, or a real 28 mm. On the other hand, slick high pressure 700x32 work very well both on asphalt and decent gravel. So your desire for two sets of wheels depends on how often you would NOT be satisfied with a compromise and would prefer more specialized tires.
In terms of compromises:
- Since the front wheel has no traction, a front slick works as well as a front tire with sculptures in almost anything.
- I once had a Vittoria Lizzard tire that was quite good for various soil conditions, including a moderate amount of hard snow. It had a smooth central section (not raised, just smooth) and sculptures on the sides, so the tire rolled on asphalt on the smooth section and the sculptures gripped in sand and snow.
One note : I have a 2000 Trek 520 (threaded fork). While there is more than enough room around the rear tire, the front area is a bit crowded. I needed a bit of creative fender filing and reshaping to fit a 700x37 Nokian Hakkappelliitta and a fender under the fork crown.
Another possibility would be to have two rear wheels and a single one.
Changing gears
Short of replacing the whole crankset with a mountain crankset (44-34-22) or with a Sugino compact (48-36-26), the best solution would be to replace your smallest chainring. Right now, you either have 52-42-30 (pre-2008) or 50-39-30 chainrings, in which case, you can easily replace the "30" by either a 26 or a 24 respectively. That way, you would keep your typical riding gears, but you would get slower easier gears for the hills.
Swapping wheels and shifting compatibility
If you want two sets of wheels, it's most likely because you want a quick swap. In that case, you will likely want each wheel to be fitted with a cassette. If the shifting needs readjustment when you swap wheels, an easy solution might be to install a 1 mm spacer behind one of the cassettes.