Assuming a quality power supply, get yourself a Radeon HD 6950 for that price range. You can soft-mod a lot of them to a Radeon HD 6970, and unlock extra performance for free. The nVidia equivalent is the GeForce GTX 570, which is also a very fast card for the price, but it's going to be closer to $300-$350. The 6950 is going to be $250-$300.