It depends at what res you game.In current games once you start getting to 1920*1050 your graphics card starts to be the major bottleneck. If you overclocked you cpu a bit more it should be able to handle even a 5850 or 5870 at 1920*1050 or greater. Although it should still be sweet at lower res too. Lots of tests actually show the e8400 keeps up with i7 extremely well in gaming.
So overclock that cpu so she flies and see if you can get your hands on a 5850!!