Installing >= 4GB of RAM has "voodoo consequences" - it'll depend on a mix of chipset, installed peripherals (especially video cards tend to grab a lot of address space), operating system, and the phase of the moon. Should work just fine, but you can find yourself with anything from 2GB to 3.25GB of memory available for the 32bit O/S. Oh yeah, also depends on the "memory remap" feature of your BIOS (if it has it at all).
Driver compatibility problems shouldn't be THAT much of an issue, most stuff is sorted out - except for stuff like old scanners, old printers, and old webcams. YMMV.
Forget about rack harddrive, install a single small-but-fast drive (36gig raptor should be enough, 74gig raptor certainly would) for your "OS drive". Partition for the systems you'll need (32bit XP, 64bit XP, and possibly even 64bit Vista for testing), and use multiboot. Works like a charm if you install in the right order (which, iirc
, would be XP32, XP64, Vista64). Use the rest of your drives in a safe
RAID mode - remember that RAID5 makes you cry even harder when two drives fail at the same time.
850W PSU sounds overkill, your system will probably be below the 350W mark in total consumption. I've got a Q6600@3.0GHz, 8gig ram, 2x74gig raptors, GF8800GT/512meg (POWERDRAINER!) and a single sata optical, and I draw like 230W fully loaded (iirc, will have to check notes to be sure). Doesn't hurt much with a bigger PSU, but iirc they're most efficient above some low-mark and beneath some high-mark; going below or above in power consumption means wasting energy. I probably waste energy with my 750W (or was it 700W? PSU.
Can't comment on the motherboard right now, it's a bit late and it's friday
- but P5E3 sounds a bit old-ish for a new system, even if it's (judging from the "3") using DDR3 memory? (which is insanely expensive atm).