Every card I know of (and i'm a technician) that is 8x is 4x compatible. If you're looking at radeons don't do the 9600. Try to find the 9700 pro. It's virtually the same card as the 9800. The main difference was marketing propaganda. You'll save bucks and have 99% the same card. If you're looking to the future in terms of board upgrades you may want to do some research on pci express. It's going replace agp and standard pci buses. The boards are already starting to come onto the market. PCI express is generally alot quicker.
Hope this helps
I have a 4x AGP-compatible mobo. I am looking to get a 128 MB vid card, and I am thinking that it would probably make sense to get an 8x AGP card. I would assume they are backwards compatible to a 4x mobo without any major issues? I would rather not update the mobo at this time if I can help it, but I also don't want to limit myself for the future by getting a 4x AGP card. I was looking at probably the Radeon 9600 or 9800, although the 9200 is a possibility too. I don't know much about Nvidias, but I would be open to looking at them too. The biggest thing is that it needs to be DX 9.0 compatible.