This depends a lot on your definition of 'not too pricey', but first you need to establish whether you have a pci express slot for a gfx card to sit in. Older cards are AGP slot, and are being phased out. If your mobo is only a year or two old, it should have pci express on there.
The nvidia 6600 chipset (256MB) seems to be a good start off point. The game Titan quest (quite new) states that it was designed & optimised for this chipset and above so it's worth looking at as a benchmark test. To run something like F.E.A.R. on high settings you'll need to spend a lot so spend as much as you can afford. What a ramble - anyway I suggest 256 MB pci express slot card and then go from there.