I'd pick up a used card if you don't want to spend too much. You can usually get a relatively cheap card from ebay. If you do buy any computer part there, make sure they take returns in case it's DOA. Also make sure to run some benchmark software on it to make sure there's no problems before the return period is up. I didn't tell you to do that.
I prefer AMD/ATI cards, others prefer nVidia. You should be able to get a brand new card for under $50, but it all depends on the socket and what card you want.
If you just have a PCIe and want something cheap, look at the Radeon HD 4650 or 4670. They're a couple of generations old, so they should be pretty cheap. They don't require external power, and they are enough for light/moderate gaming. I also had good luck with a PNY 9800GT low power card. If you want a room heater for your computer, and a relatively quick and cheap graphics card, I've got a 4870 that I'm trying to get rid of. PM me if you're interested.
Running on a 4-slot toaster @ 60Hz