Tom’s Hardware has an article up titled “AGP Platform Analysis, Part 1: New Cards, Old System” that takes a look at whether it is worth upgrading your video card on an AGP-based system or whether your old system will just be a bottleneck on video performance.
Among the cards used in the article is the Sapphire x1950 Pro AGP that I just installed in my own aging setup. The conclusion of the article seems to be that the higher the resolution you run, the less likely your system is to be the bottleneck on performance. Since I insist on running at the native resolution of my Dell 20″ LCD, which is 1600×1200, I was pretty happy with that answer.
Tom’s Hardware also has their regular “Best Gaming Video Cards For The Money” article up for January that covers both AGP and PCI-E cards.