If you want a killer gaming machine, IMO you should build one from scratch. You would need to upgrade your ram, cpu, and video card. If you swapped the TNT with a new radeon, you would only notice a minor improvement. Your ram and cpu could not keep up with that card. Definatley do some research and build yourself a new rig...if your short on cash save up for a while, you want your machine to rock.
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Kill a man and you're a murderer;
Kill many and you're a conquerer;
Kill 'em all and you're a god.