They can be. but, you also aren't going to be looking for a brand new top of the line fancy one. You'll be looking for a model a few years old. You will need a PCIexpress slot and the appropriate power hook ups for a GPU. Most computers should have these.
It can be a bit of a power hog so you may need to upgrade your power supply but, I always had good luck with a GTX460
http://www.ascendtech.us/nvidia-gefo...ent=pricewatch
I also had good luck with a GTX260 so this may work for you and would be less of a power hog
http://www.pricewatch.com/price/vide...ce_gts_250_1gb

FYI I am currently using a 4 series card (GTX480) and they are currently making 7 series cards so, the 4 series are still decent cards.