7600 gs is the best after taking into account the rarity and price of the faster cards. (The best AGP cards are usually stupidly expensive.)
I remember that's what I used to use (before upgrading to the 4670) with my dual overclocked Tualatin before I got bored with and gave it away.
I was able to run Skyrim, Crysis, fallout New Vegas, and Bioshock 2 mostly playably under Windows 7. But I had a board that supported 4 gigs of Ram.
(The 4670 is only marginally faster)
6200's are really cheap. The 512MB full height variant is pretty good.
(I think the low profile version is only 64 bit, while the full height version is 128 bit. Could be wrong though.)
7950GT's are very rare, but would be the best Nvidia card.
The 1 gig HD4670, or 3850 are basically identical in performance and they are the best cards on the AMD side, but rare and expensive.
Another option is pci to pcie conversion, which is kinda stupid and silly, but you would be able to install something like a GTX 260 which I consider to be one of the best price per performance cards that exists today. You can pick these things up for like 10 bucks, and they still handle modern games pretty well. (It'll be super bottlenecked by the slow pci bus, and the translation from PCIe packets to a PCI signal, but it used to work for me on my dual Tualatin.)