Is your CPU OC'd any from the 2.4 GHZ stock?, if not you would probably be fine with a GTX 460 or 560, If you can hold off a few more weeks. NVIDIA should be releasing they're new Kepler cards, unless the rumors are true about them holding off and releasing 700 series, if thats the case then we may not see new stuff till summer. Now if you want a AMD card the 6850 (Sapphire or XFX) is a good one or the 6870.
its overclocked to 3.2ghz. arent those gpus too expensive? i was looking for something cheap..