For $450 you can buy a raleigh back alley and not ruin a nice ten speed. There are also other pretty descent complete fixed gear bikes that can be had in that price range. The back alley is really nice for the money. Comes with DP-18s, flip flop fixed/free rear hub, front and rear brakes, it's even got built in chain tensioners like the rush hour. Reynolds 520 frame.
Raleigh Bicycles » Back Alley if you really want to convert the bike you have the DP-18s are great rims in my experience, laced to some formula hubs or something like that. I'd recommend them.