First, be sure you have at least a 450W PSU with the necessary PCIe power connector(s). 500W if buying a factory OC'd model. And no cheap PSUs. The +12V rail should be rated for at least 35 amp @ 420W. That is assuming the rest of your system is about average. (Should state the system for verification)
Next, be sure the card you choose isn't too long for the space it fits into in the case. Measure from the expansion slot cover to the first obstruction you come to. Usually the drive cage.
Last, check that a double slot card will fit if the one you have now happens to only be single slot. And we are talking full height cards, not low-profile.