Based on my assumptions about drives, fans, etc. the PSU calculator at http://www.extreme.outervision.com/PSUEngine suggests 643W will be sufficient.
A quality modern PSU has full range active PFC (no little voltage switch) and some level of 80+ certification for efficiency. Seasonic, Antec, Corsair, XFX, and Enermax/LEPA are among the better brands.
In your place, I'd be gnashing my teeth for missing out on the Seasonic X750 for $119 last month; by the first of the year there will likely be another good deal or two.
I've got a 550W Truepower New in my backup PC, and a Seasonic X-560 in my primary. The latter may get replaced with my older SG-650 if I decide I want another video card; for now I can't help but think the SG is being wasted as my current "bench" PSU.
For a system using a single Radeon HD 6990 graphics card AMD specifies a minimum of a 750 Watt or greater power supply. The power supply should also have a combined +12 Volt continuous current rating of 50 Amps or greater and have at least two 8-pin PCI Express supplementary power connectors.
When the performance micro-switch is set to use the Extreme Performance BIOS the card alone will be drawing up to 370 Watts (30.8 Amps) from the +12 Volt rail(s).
Overclocking the Radeon HD 6990 with the performance micro-switch in the Extreme Performance BIOS position will increase the power draw and temperature even more.
A reputable brand 750 Watt power supply unit like the non-modular Corsair Enthusiast Series TX750 V2 (CMPSU-750TXV2) or the modular Corsair Professional Series HX750W (CMPSU-750HX), with their +12 Volt continuous current rating of 62 Amps and with four 8-pin PCI Express supplementary power connectors will properly power your system using a single Radeon HD 6990 with ample reserve capacity to handle any overclocking of the CPU and graphics card you may want to attempt.