FYI, the D820 series only sees 3.3GB of RAM regardless of a 32-bit or 64-bit OS install because ~700MB of RAM is used by the system BIOS for PCI devices and such - limitation of the Intel chipset the systems use. In other words, just install the 32-bit version probably so there's less hassles...
I registered here just to say thank you Lapino. I have an ASUS P5B-E as well and 7 wasn't powering off my computer either, just a black screen and the fans stayed running. The 1394 controller being disabled worked like a charm! Thanks! I think it may be time to look at replacing this setup...