Another thing to be worried about is if it will fit in your case. Will the layout of your case fit the Graphics card? I actually had a case where the layout was weird and wouldn't fit a certain graphics card, and I had to downgrade.
And make sure it'll fit on your motherboard as well. These graphics cards are HUGE, and sometimes there could be a random heatsink on your motherboard that won't interfere with the card.
And last but not least; THE POWER SUPPLY. Make sure your power supply has enough wattage to support this. If it's the stock power supply [the one you bought with the pre-built computer], then I highly recommend you upgrade to something like 400 or 500W.
All that what Galaki said about the PCI slot are all the things you should make sure about your computer. Here is a picture of a PCI-Express x16 slot:
PS: The two white ones on the left are normal PCI slots, usually outdated these days. The small black ones are PCI-Express x1, which are used in some things [for example, I use it in my HDMI Video Capture Card].
"Being single is easier on the gaming life, and the wallet."
