also usefull to know would be wether you have a PCIe x1 or x16, most of the boards i've seen with only 1 pcie slot were only x1 (could be wrong though)
Select a PCI-E card on your budget...but also consider the facts. Nvidia has the best cards out (basically). The 7000 series of Nvidia has some great values. The 8000 series gets good at the 8800 GTS and up. Take a look at those on your favorite online stores and compare the #'s.