Just by looking at the specs, I'd say that the ATI card would be a touch faster, due to higher clock speeds. However, the 7800gs has 4 more pipelines that might make up for the slightly slower clock speeds.
According to the link that Sterling14
provided (gotta love TomsHardware!), the ATI card is slightly faster (both cards were AGP too), but only a touch under 8 FPS. Basically, it's now do you want Nvidia or ATI. I think that you'd be happy with either card.
While a smaller card might save you some cash, I think that the difference in FPS would make it worth while to purchase one of the cards that you listed.
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