Forget 7000 series GeForces in AGP. For one, there's only a handful, and the one card that shouldn't cost an arm and a leg, *does* cost an arm and a leg, because it's the only one available in AGP (7600GT). The 7800GS AGP card is crippled compared to the same card in PCIe. It's not worth the extra money to get anything that fast in AGP. They cut back the number of pipelines, they cut back on half the features, just because it's AGP.
Stay around $100 if you're sticking with AGP. Any more than that, and consider moving to PCIe. That involves a motherboard change.
EDIT: Airspro, the X800GTO is one of the better ATI cards. I've only heard good things about it. However the X1600 is pretty good as well, and $30 less. Side by side, the X800 outperforms the X1600, but not by too much. It's a matter of how much you're willing to pay on a card that has to last you many years, and whether you're going to switch to PCIe before the operational life of that card is over.