Mainly PSU, mobo and case.
1) Make sure you have enough wattage:
http://extreme.outervision.com/psucalculatorlite.jsp
2) Make sure your mobo is Crossfire compatible (look at your specific product specs on the manufacturer website).
This means having at least two PCI-e x8/x16 ports with the correct spacing.
3) Make sure you have enough PCI slots in the back of your case to ensure a proper install of the second GPU.