I agree with some of the other posters. You don't need a new cpu. I have the same one and it does just fine. I oced it to 3.6 GHz, but I only lost a few fps when it was running stock. So you will see some improvement if you oc it, but even if you don't it won't ruin your experience, its up to your preference.
As others have suggested, a nice gtx 460 would be perfect for you, sure the 560 ti and 570 are much more powerful, you really don't need that much horsepower. Get 2 GB more ram and move to a x64 bit os to fully utilize it. Windows 7 x64 being the obvious choice here... well as far as gaming is considered anyways, I use Ubuntu for everything other than gaming
. Get a new psu if you can. You really only need something between 500w to 600w for a single card. If you can get something higher, say around 750 - 850ish watts, then that will open the road for sli if you want.
Other than that, you are all set