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Can you give more specifics about your system and what benchmarks you used for speed?

Sounds like a video card issue to me. go somewhere like and newegg and form the computer you bought I would assume you arent going for super-high end and get something like the 7600gt If you have PCI-e, or an 6800 if you are on an agp platform, If you don't know what either of those things are, I would reccoment taking it to your local repair shop, or at least read up on computers before you start opening your computer.