A good card that is actually pretty cheap is the ATI Radeon HD 4850. That's the card I use, and I can tell you, it runs every new game out there really smoothly. I tested out COD 4 and Left 4 Dead, and they work really well on high settings, as well as Tales of Monkey Island and Sam and Max. It doesn't need a lot of power to run (about 450 W is the recommended amount) and it runs in any slot from PCI Express 1.0 onwards. Just make sure your power supply comes with a 6-pin PCI Express connector, otherwise, it won't run.
"There was an old farmer who lived on a rock,
he sat in the meadow just shaking his..."-The assumption song. A random youtube search can bring on a big laugh.