arzbhatia is correct the Extreme Models are the best of the 775 cpu's, however, a G0 tepping q6600 are readily available on ebay, and are great overclockers, if you can go that route.Not sure if your mo-bo has many overclocking options though. My G0 is steady at 3.0ghz and with a 560ti I can run all games bar BF3 maxed out. BF3 is on high at 16 x 10 and I'm happy with it. (Naturally my system is holding me back) I will be atempting to sqeeuze more out of my G0 Stepped Q6600, because I can't afford a system upgrade right now.
Anyway, for your original question, anything above a 5 series NVidia Card or maybe 6850/6870 ATI, your exisiting processor is prob gonna hold you back. Try overclocking it a little, and with a newer card, you will see an improvement in games, thats for sure.