Well I am a bit off on lower tech but my father needs a pci vid card. Found an old emachine and for little money it can be far better than what he currently has. He has a really old athalon xp system running around 1.3 and the emachine is running a celeron with a 478 socket. I can get a p4 for cheap and 2 gigs of ram really cheap just need a vid card so he can watch video and play some older games till he can save up for a new system.
Get a 8400gs, 4650 is too powerful for a p4 rig that doesn't game
ubernoobie is right. No matter what you get you wont get good performance out of a PCI slot anyway--so just get whatever is cheapest.
The important thing with an older system isnt that the graphics card will be really powerful--its that the graphics card takes some of the strain off the rest of the system, frees up more ram, etc etc.
(are you sure you cant find an old P4 system with at least an AGP slot?)
Yeah, I have seen emachine variations with NO AGP or PCIe slot. Pretty amazing.
If that's the case, then yeah, I'd just get whatever's cheapest. Regular PCI slots transfer data something like 50 times slower than a PCIe x16 slot, so you're going to be horribly constricted no matter what you do.
Personally, I think it's a lost cause to invest money in a machine like that; it's only EVER going to be any good for iTunes and the Internet. I'd just see if you can find an old P4 machine for $100 or so that DOES have an AGP or PCIe slot, then get a 4650 or 4670 to put in that.