I personally question why you might want to add a second card. Is it that you want more than 2 displays at once? That's the only potential benefit I might see in adding a PCI video card alongside your AGP one. Another alternative is that one might want to install a 3dfx card to play some old Glide-only games, (like The Elder Scrolls Adventures: Redguard) but given that you said "GeForce" and there are plenty of good, free "Glide Wrappers," there's no reason for that.
Currently, the most powerful PCI card from nVidia is the GeForce 6200. The most powerful from ATi is the decidedly stronger Radeon X1300. Neither is terribly good in their PCI form, though; it's a far cry from AGP, which is not merely faster, but a superior architecture as well with far less latency.
When it comes to AGP, the most potent card is the Radeon X1950pro, with the best GeForce being the 7600GT, or if you count factory-OC'ed cards, one of Gainward's 7800GS Bliss cards, which is technically a 7900GT/7800GTX labeled as a 7800GS.