Hey,
1. i just tried 2 more combinations:
9800GTX+ & 8800 GTS 640 ... BSOD ... (swapping not possible)
GTX 480 & GTX 570 (using a Ghetto dual PSU Setup :D ) :
Worked fine, but the 480 lost Memory, Bandwith ( & alot of other crap)
GTX 480 was at full GPU load, while the 570 was at...