Use this tool to determine the correct wattage needed for you PSU:http://www.extreme.outervision.com/psucalculatorlite.jsp
Then checkout this link:http://www.tomshardware.com/forum/324368-28-computer-power-supplies-guide
When I buy a PSU, I take the minimum wattage from the power supply calculator and double it. (most PSU's are most efficient at about 50%-60% of their rated load). I then pickup something with a single +12v rail, that's 80+ certified, usually from Seasonic or PC Power and Cooling (but there are many other great brands, always check the reviews)
Make sure that your selected PSU has the proper power connectors for your ATI 6990, probably 2x8 pin.