I too am running a new system:
AMD Phenom x6, 8GB DDR3, ATI Radeon 3000 (on board), ATI Radeon HD 5750 (external)
and also getting this annoying problem.
The nags usually begin with video or media visualization on second monitor then eventually turns off Aero with a message about not having enough dedicated video memory (DVM), which is BS according to Process Explorer
It show's typically less than half the DVM is actually in use.
I recently turned off "User Account Control", and it seems to have mitigated the problem, even when pushing the GPU usage to the limit. But I'm still not sure if this fixed the actual problem and I also don't like disabling ALL the safety nags (some are good, imo).