Dual core games are just now coming out. Although you would get more performance from a higher end single core in current games, a dual core cpu handle them fine. A lot of new games will use dual core, UT 2007 is going to as well as Elder Scrolls IV: Oblivion which comes out March 24 (we hope).
Dual core isn't a waste, but the FX-55 still has quite a bit of life remaining so you should be fine.
single threaded games = single core performs better
multi-threaded games = dual core performs better