Agreed on the clean install - always a prefence if that is possible.
For a desktop, my favorite approach is to add a new SATA drive, about 320gb or so (I bought a handful of WD Caviar Blue for $30 each.) Do the clean install to that, then leave the old drive in the machine as-is but unpluggd, "just in case."
For a variation on that approach, I will do an install, then tweak all of my setting, do updates, and install "core" aps that I always use, like PS and Office. Then use something like Acronis to create a clone drive that I leave in the machine, unplugged and with no power. Makes it easy to do a restage if needed in the future.
I have done the same with other sever boxes when testing OS'es. Much cleaner than dual boot. Or leave them plugged in and select boot order frm the BIOS at boot.
Win 7 is clean, fast, stable on 6 machines so far. I also like the Media Center for Netflix, etc. Nice upgrade there.
64 bit works well on a Pentium D w/ 4 gb, so should work fine on all more recent boxes with at least 4gb of RAM.