need help w/vista 64bit

ok so I'm having some issues trying to get my new computer up and running. Here are the specs
DFI LP UT NF680I LT SLI-T2 motherboard
intel Q6700
8gigs pc6400 mushkin epp ram (4 2gb sticks)
2x 8800gts
500gb sata hd
825watt PC power & cooling psu
running vista home premium x64

ok so I knew about the ram issue in vista x64 so I installed vista with only 2 gigs of ram in my mobo. Once windows was done installing I updated my bios to the latest one from dfi, then shutdown and put all 8gigs of ram in my motherboard and tried to boot just for the hell of it. With all 8gigs of memory in my computer would post and recognize all 8gigs of ram, but then when it would go to load windows it what blue screen of death on me. So I went back to 2gigs of ram and downloaded the windows update regarding the memory issue for vista x64, shut down and put all 8gigs back in my comp and turn it back on, now it doesn't even post. So I took out 2 sticks of ram and have 4 gigs in, and everything works, I disabled memory caching in my bios and got my comp to work with 6gigs of ram. At this point I thought maybe I should run memtest on my ram, so I did this over 2 days testing 2 different chips at a time for about 10 hrs each and they all passed and everytime I open up my case to try and fix it I alternate what slots the ram sit in. I already called dfi and they said they'd call me back.... its been 3days. so if someone could help me out here it would really be appreciated.
Thank You