For judging the fit, you'll want to look at the measurements of the card itself and your case. If it's a semi recent case it should have a max GPU length listed. If it doesn't you can manually measure it by measuring how deep in to the case it can reach before encountering something that would stop it(case side, fans, hard drive bays, etc). If the card isn't any longer than your motherboard, you're typically safe as well as the case as to at least accommodate the width of your motherboard.
Having enough power... not sure exactly what you mean. If you mean enough power to run a game, look at the minimum/recommended specs for the game and that will tell you what models it needs to run. If you mean if the computer will have enough power to run it, the card will state how much power it needs to run and your PSU will have it's own power rating. You'll need to compare the two and see.
If it's compatible, just make sure your motherboard has a PCIe x16 slot you can use and you'll be fine.