Dammit :slapass: I can't remember, did you try memtest86 in that rig? http://www.memtest86.com/ Surely it has to be RAM or motherboard related to halt after running fine for a few days? You've probably tried this too but disable anything you don't need in BIOS & strip out any cdroms/floppy/sound cards etc.
Maybe it's worth trying linux on there & see how you get on :shrug: I've just put fedora core 5 on my xp1800 & it's running really nicely and is getting about 90ppd, which is around what I'd expect.
I'm not sure what a 3700+'s worth but I'd keep it for a future rig, any chance of finding a real cheap 3800x2 over there? - I was tempted by a stupidily cheap 939 x2 deal from a UK site, but the next day they were gone; had to go C2D instead, shame ;)