from what ive read, 8800gtx runs at ~68C idle, so if ur idleing at 74 without the fan speed increase the card is definately running hot ! and seein as your only crashin when ur cards speedin then that points another finger at ur card temp...
Have u got access to another system that you can test the card in ?
Besides that make sure that ur CPU and RAM are definately happy with each other i.e if ones runnin higher, clock it down and make sure youve enable the timing flexibility option in the BIOS if there is one. Although im sure youve thought about this when u bought the parts.
OH, could it be that your card is using too much system ram ? if uv only got a Gig in there.. your card in 768mb.. if u can lower the PCI-e apeature size (how much system ram your PCI-E card uses) then that may solve it.
If that fails I would contact the card Manufacturer and tell em whats goin on (no need to mention how uv tryed to rectify it, just tell em u noticed its runnin at high temp and its crashin when ur tryina benchmark)... could be a common fault.
Apart from that.. i feel for u man, u must be fkin gutted !!!
P.S. can u check ur PSU as Ste suggested ?