if you're taking off the "free cassette" on the old wheel and replacing it with a free wheel single sprocket then you'll probably need to re dish the rim and space the hub over a bit to get the correct chain alignment.
if you're going to fixie it then the same applies but remember that a screw on fixed cog will try and unscrew itself when you slow down the cranks with your legs. not so much of a problem if you keep the brakes fitted though.
check out london fixed gear or somewhere similar for more advice.
have fun.
get a half link chain to help with limited tension adjustment.