I'd suggest an Nvidia 6600 for $85
it has 8 pixel pipelines, and outclasses all cards mentioned above
AGP 8X cards are backward compatible with 4X, so that if your motherboard doesn't support AGP 8X, you can still use it if you have AGP 4X
basically, this just means the bandwith to the card is not as much. however, even a 6600 won't be hindered much, if at all by the bandwith of AGP 4X against AGP 8X.
1.1GB/S (AGP 4X bandwith) is enough for a card with 8 pixel pipelines
2.2GB/S (AGP 8X bandwith) is enough for a 16 pixel pipeline card
after that, there's PCI-E 16X (16GB/S) which gives lots of headroom, but the most pixel pipelines in a video card so far is 24