What sort of card did you have in before the upgrade to the 7900?
IF it was ATI did you uninstall all the ati drivers and settings?
I am almost sure you have a bad ram chip. One of them is either not getting cooled properly, or worse just ended up getting corrupted.
i7 930 @ 4.2Ghz
GIGABYTE x58 UD7
Kingston HyperX T1 Series 12GB
GIGABYTE 7970 (1280/1800)
OCZ Vertex 2 100GB SSD & VelociRaptor 300GB