The fastest PCI videocards available at this point are in the Geforce FX series, or in the DirectX8 Radeon 9200 series. No further PCI videocards are planned, because there simply is not a large market for them.
If you want to be able to play games, a PCI videocard will suffice for games made in 2002 or before. A Radeon 9200SE for PCI will suffice at medium graphics settings for 2002 and earlier games. However, if you want to be able to play newer games than that, an AGP or PCIe card would be much, much better.
It depends largely on the games you want to play. Upgrading your motherboard would allow you to use newer AGP video cards, although that might not be possible depending on whether or not your system uses a proprietary layout (e.g. Dell).