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The 512MB 4670 offers tremendous bang for buck whilst using very little power. That 8800GT is going to be so hampered by its lack of RAM its not really worth considering.
the 4670's 128bit Bus will hamper that lol. Benchmark wise the 8800GT 256Mb beats the 4670. Get whichever is cheapest I'd say, the 8800GT is a superior card and the 256Mb of RAM will only effect it massively when you get to 1680 x 1050 with AA and at the point the 4670 would be no good anyway. Obviously the 8800GT 256 is not comparable with the 512Mb version due to the RAM and will be significantly slower, but 8800GT 256 is a bit better than the 4670 for games.