Also have you considered an SSD? Given that you're already spending a good amount, you may find that investing in an SSD for booting and keeping your frequented programs will make everything a ton faster, Hard Disk Drives really slow down a lot of things.
I have a Radeon 6870, it can reasonably play any game I throw at it with max settings and it only costs about $150. You could get a $200 GPU, and use the extra $100 for a 120Gb SSD that you could use alongside the HDD, you would not regret it.