Actually, i was quite surprised to find this after a little looking...
http://www3.pny.com/8400-GS-512MB-PCI-Low-Profile-P2679C269.aspx#Specifications
... which might be worth looking at if you can find it cheap (i.e - on ebay). The 8400 chips are solid entry-level ones that are becoming popular onboard graphics choices now, as they can reportedly run Call of Duty Modern Warfare if you don't mind the framerate getting a little low (on lower resolutions, natch).
The way I see it though, the problem isn't the graphics chip itself - it's the PCI slot interface that would be the limitation. The reason they change slot standards is that each kind of slot has an upper limit for how quickly graphics information can pass through it. You could, in theory, put the most powerful graphics card in the world into a pci interface, but it would be a waste because it processes graphics information much, much faster than this information can pass through a PCI slot.
Unfortunately, I can't find any benchmarks or reviews for the 8400gs PCI card, so I genuinely don't know how
![G o o d [Good]](/emoticons/g_o_o_d.gif)
it'll be for games. It looks like it's designed for media PCs, but it's worth a go if you absolutely must buy a PCI card.