this website http://www.peak.org/~fixin/ has a program that will tell you what combinations of gears you can use on your frame and have good chain tension. I've never tried this but it seems like it would work at least until your drivetrain gets worn.

If your interested in turning the bike into a freewheeling singlespeed you could use your cassette wheel and put a BMX cog and spacers on it. You could the program listed above to figure the right gears to use on your frame or you could use a chain tensioning device to take up the slack. A chain tensioner will only work on a freewheeling bike. You can't use one with a fixed gear.

Take a look through Sheldon's site. http://www.sheldonbrown.com/fixed-conversion.html