Installing new video cards (or other hardware) in a pre-built retail system is always with risk, especially for HP and Dell systems. If you are buying a low end gaming card, buy your video card at a retail store in person with a 100% refund policy. For example, for cheap gaming you can buy a Zotac 650 ti video card at BestBuy. Try it out, if it doesn't fit/work, just return it for a 100% refund. Higher end video cards however are almost impossible to buy at retail stores with 100% refund policies.
However, if you are buying a high end video card, you should be willing to either completely build your own system or buy one of the high priced pre-built gaming systems (Alienware, FalconNorthwest, OriginPC).