You could just stick with the 6800 AGP line, it's a bit cheaper than any similar/better performing parts in the 7-series.
$125 after rebate for a 6800XT (apparently the PVT42KUDE3 model is only a 128bit memory interface, and is more expensive with the rebate, so if you searched and find that keep it in mind).
A 7600GS would also probably be a good performer, even though it has fewer pipelines, but it costs a little bit more.
There isn't a lot of value in trying to go from such a high-performing card to anything faster, while still being limited to an AGP system, although of course your current situation is essentially zero performance. It's up to you whether the somewhat higher performance is worth the money. A 7600GS would not always perform better, but it may be only 20 dollars more expensive, whereas a 7800GS will always be higher performance but is 140 dollars more expensive.