You have to make sure that you are getting the right video board connection for your PC: ie PCI Express
You have to read your power supply to make sure that the video card is not above the wattage of your power supply.
If you buy a video card off the shelf at BestBuy, you can browse the various specs, and pay $129 for a video card that sells elsewhere for $45 through electronics suppliers.
As I have a 300 watt PS, I was limited to the Galaxy Nvidia GT 420 ... and I paid way too much for it.