I've seen these wheels recommended before, and they are pretty affordable. If you're really strapped for cash, just get the rear wheel, if you can afford it, getting both will probably be a significant upgrade to the bike:
http://www.wabicycles.com/GXwheels.html
If you've got the know-how and/or patience, you could also build your wheels up yourself and probably save a few dollars, but it may or may not be worth it, as it's a lot of work if you've never done it before, and you need a wheel truing stand.
As mentioned by many people, you will likely also need to make some sort of change in the crank/bottom bracket area to make your chainline correct.