Yeah, the good ones like Nvidia GTX 570 are awesome. I think it's got pixel shader 4? Or something, which is great. Also it's going to be good for a while, now. So you might as well buy it.
Not sure how much it costs though.
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Don't buy a high end GPU in a shitty case with a crappy CPU. High end GPUs produce a lot more heat. Small, poorly circulated cases would get fried. 2nd, your CPU still handles the game engine. So while a high end GPU will take the graphics load off your CPU and lets it handle the engine, if it can't do that well, then you have wasted a lot of money.