Both manufacturers are fine. Depending on your budget either one has a card best suited. There is no out and out best. You will get better answers in the gfx card part of the forum.
But really, they’re all as good as each other as the wolf guy said. Though if your waiting for the latest generation, then AMD looks to have a possibility of being far better than the G80 although the G80 is available today.
If you’re talking about the last generation then unless you want to go SLI in one way or the other then you’re talking about ATi being the better.
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