I'm trying to make a decent computer for Diablo 3. Is it possible for me to buy an old dell desktop and upgrade the processor, graphics card, and RAM in order to have a computer that can run it well? Any help appreciated.
You maybe could... It depends mostly on the power supply and whether it can dish out the power to whatever you want to put in it, and whether the BIOS is locked to the original CPU frequency/multipliers (and/or supports the processor you want to use).
I personally wouldn't do it unless the thing is under 200$ and relatively modern (DD3, socket AM3/2 or the intel SB socket, Sata2 or better, etc), all you are really getting is a proprietary Dell Case/Mobo/PSU (all of these could end up being useless depending on dell's design choices for that PC), some RAM (also useless if it's DDR2), an HDD, and an optical drive (limited value if they are IDE instead of SATA and you end up needing a mobo).
All of that stuff could be bought for ~300-400$ without all the potential gotcha's.