I have to agree with what's been said, for your older computer, a top of the line video card isn't a good match for your CPU. Unless you plan to upgrade the CPU soon, which will probably require a bunch of other upgrades, like motherboard, memory, etc.
I would look for something cheap in the Aces High Classifieds, from a GeForce 9800 to a GTX 260. If you can't find anything, then Newegg has the 550Ti for $100. I wouldn't worry about the Pci-E slot, as long as its a x8 or x16, it'll work fine regardless if its a 1.0, 2.0 or 3.0. Make sure you put it into the slot designed for video cards, by all rights it should be a x16 slot.
As for the memory on the video card, unless you're running a 64 bit OS, you want to keep your video card memory down to 1MB or less. Another reason to not go crazy with one of the new 660 Ti s.
Newegg has a refurbished GTX280 for $100 at the moment. I'd be wary of that, because the big second number means its a power-hungry beast of a video card. You'll want to be sure of your power supply first. Otherwise it seems like a pretty decent deal, but I'm not sure how it compares to a 550 Ti in Aces High. Often with Nvidia video cards, the second number is more important than the first, so for example, a GTX480 will outperform a GTX640. But as the generations get further apart, its harder to say for sure.