Make sure your PC has enough space to house the card your buying, and make sure you meet the power requirements. (You will need a PCI-Express x16 Video Card.)
If you buy a card that's 9.6 inches long and you can only house one that's up to 8.5 inches long, obviously it won't work because it won't fit.
AS for powersupplies it's best to get a certified powersupply so you don't waste energy, generate unnecessary heat, and be able to power your components reliably.