Re: BFG 6800GT listed at 11.9MHz GPU clocks
That PSU is likely the culprit. Newer CPUs and mobos draw their power almost exclusively from the 12V rail, as opposed to speading it on the 5V, 3.3V, and 12V rails. As such, with a newer system (even with that mobo's 20-pin connector) you'll want at least 22A on the 12V rail for your PSU.
The power draw on the 6800GT is roughly the same as or more than a 7800GT, and that's recommended to be 12V@20A for a stock clocked card.