I had probs running a clean install Win 7 x64 Ultimate - Asus mboard, AMD quad-core, similar setup (but nVidia graphics' card - am not a huge ATI fan). Would get inexplicable errors reading CD-ROM and other install issues, including BSODs. Did you have any issues with any software installing on your machine? Any BSODs? This was particularly maddening to me since I had zero issues with this setup using Vista x64 Ultimate SP1. I too thought it wasn't memory after a year w/o issues in Vista. I upgraded and reset Asus BIOS (pulling out the jumper and resetting), set the memory to the 2.1 volts suggested by Corsair, but this didn't stop the issue either (I had 2 DVDs of the 7; it installed fine on my laptop but neither worked on my desktop system so it wasn't bad media either). When I went on the Corsair tech board, they had specified that this particular motherboard (and I assume others as well) while supporting 8gb in this config, 8gb did not pass their "certification test." 4gb was the norm. I pulled out 2x 2gb sticks, down to 4gb total (all Corsair CM2x2048-8500C5D matched sticks), reduced memory speed to 800mhz and 2.1 volts. All installled ok and everything is "stable" now. Memory tests in Windows 7 come back w/o error.
If memory (mine this time) serves, this memory issue seems to crop up with every OS that MS releases, stretching back to the Win95 days. Even though your memory worked perfectly in the same config with an earlier OS, don't assume that it's not memory - even Vid cards, too. I've made or modified my systems since 1984 and while prices have come down on the parts tremendously the time you waste hasn't and the help you get is hit or miss. My Win 2003 R2 server x64 is running 8gb w/o issue but not this OS.
Hope this helps and good luck.