It would be easy to keep your 7-speed setup and make the move to a freehub:
1) Find an 8/9/10 speed 26" (559) wheelset. (Or if you're mechanically inclined, you can unlace your rear wheel's spokes and replace the freewheel hub with a freehub model.)
2) Pick up a 7-speed cassette. Niagara Cycle lists a bunch of 'em on their web site:
7-speed Cassettes.
3) Install the 7-speed cassette on the 8/9/10 speed hub with a
4.5 mm spacer behind it.
...but you could also make the jump to 9- or 10-speed while you're at it.