For a gaming system your video card will be king. Processor, schmocessor. You'll notice a world of difference between a 6800GT and a 7800GT or 7900GT, whereas if you upgrade to a 4400+ CPU (as an example) you'll notice somewhere between little and no difference in your games.
The 7900GT is supposed to be around $300, and they should be available today or within the next few days on newegg. Performance is supposed to be the same as a 7800GTX, which is more expensive. The 7800GT is what I have, and am very happy with, and can be had for $260 on newegg right now. That price should be coming way down soon as well.
But seriously, a 7900GT and you'd be stoked. I'm not really sure how to exactly quantify the difference you'll see, but if you have at least a gig of RAM then with a 7900 you'd be able to play FEAR and BF2 at max settings and get excellent frame rates. Your new card would also support high settings at a number of different resolutions, depending on what kind of monitor you have.