The 7900 have more video memory ... could be the reason.
Could you mix 5800 and 7900 ?
Explanation :
Theory : the BIOS has to address all the video memory ... with 8 58x0 (8x 1 GB) was still ok ...but with 79x0 ... 8x 2GB (maybe 3) ... it is too much to handle.
Worth the try... that would explain the different behaviour ...