I started a thread with the same problem with dual HIS 4870x2's, intel i7 965, 12gb ram, wd 300gb 10,000rpm hdd, 1000w modular corsair PS, Asus p6T mobo, windows vista X64 and cannot do anything to get it to boot once the drivers are installed.
In the mean time i just did a fresh build Quad Core Q9550, Asus Rampage mobo, Thermaltake Toughpower 1200 watt PS, (great PS btw, better than the modular Corsair), 8gb DDR2 1066 Corsair Ram, wd 300gb 10,000 rpm hdd with a single Sapphire 4870x2 and windows Vista X64
Now on the PC that will not boot, i have tried updating the bios, using only 1 card in each PCI 16x slot, reinstalled Vista 64 so many times and still cannot get it to boot. Like you said i can boot into safemode all day and fool around with things but the minute you install any ATI drivers and reboot it hands and BSOD's.
The top PC i purchased prebuilt from IBUYPOWER and has not worked since i got it. The bottom pc i built from scratch from newegg.
The only conclusion i can come up with is that the P6T (i7 board) is both SLI and Crossfire capable where the Rampage board is only Crossfire capable. Is it possible that this is causing a problem with ATI?
Are you running the GFX card on its own rail of power or is it shared with the rest of the CPU? Are you having to use one of the Thermaltake reducer cables so that you can have the 6 and 8 pin connector plugged into the GFX card? Or maybe the HIS cards are all crap.... I am gonna either try and get a different vendor or unfortunately have to go with SLI cards on the i7 CPU.
i might try and swap the HIS and Sapphire cards to see if they will work in the others computer... This would definetly tell if the HIS card is crap or if there is something wrong with the PC itself.
I hope that some of my problems may shed some light onto yours and possibly help you out
Frank