I had the same problem when trying to Install Vista Ultimate x64. My system is EVGA 680i LT Motherboard, 4gb Corsair XMS (PC6400), Core 2 Duo E6600, Geforce 8800GTX.
I could install Vista initially, but upon reboot I would get the blue screen with the message IRQL more or less not equal. Figuring it was a memory issue I pulled 2 of my 1gb sticks of memory out and reinstalled vista again. It then proceeded to install without a hitch.
Thinking that the memory was bad I got it replaced put it in and had no problems, that is until I reinstalled Vista again. I got the same blue screen and error message. So again I pulled out 2gb of memory and it installed fine. After setup was complete I put the memory back in and have no problems.
Not sure if my experience helps at all, but maybe it will for someone else. Goodluck getting it figured out.