I use the ASRock Dual VSTA motherboard... E6300 conroe processor...
Anyways.. I upgraded from a Nvidia 6200 AGP to a 7900GS PCIe. Cost me about $95 with shipping and all.
My MB also only has 4X PCIe, but it works flawlessly. Frame rates rarely drop below 60, even with lots of action, fire, etc...
This is at full graphics settings, 4XAA, 1280X1024 and 1024 Texts (Hi-Res Pack).
I'm sure everyone is going to suggest an 8800GTS, which I'm sure is a fine card, but not worth the cost as far as I'm concerned... at least for my needs.
Don't let the 4X limitation discourage you from going PCIe.