Power Off on OS load with G-card...

Hardware:
CPU 6000+
2x2GB 1066 Gskill
Enermax 625w 80+ certified PSU
ATI 4850 (tried 980GTX before)
Gigabyte M750SLI-DS4 Mobo
various harddrives.
XP Pro sp2


I've got a problem I believe is motherboard related, I can't seem to find anything else to blame.

My problem is when I power on the computer (with a graphics card installed in the PCI-E) the computer will shut down while starting to load windows.

Solutions tried:
Updated Bios
Reformatting (also switched the OS to different drives in case of bad HDD)
memtest ran fine for 8 hours
various CPU tests
PSU voltages are fine
Temperatures are low

When I take the card out the computer runs fine on on-board video. This also happened with a 9800GTX which I thought was the problem so I sent it back and got a 4850...still same problem. With the 9800 I used to at least load windows..sometimes and be able to game for a little bit but would still eventually shut down at certain times.

Only thing I can think of is MOBO.