Just buy Celeron 860 from Dell, and replace the CPU with something better (if you even have to). Many standard desktop ATX boards won't fit in a 1U case, even if it's an ATX case. I stuck a Gigabyte motherboard in one, and I literally had to rip the headphone ports right off the board

It's still running to this day though!
I'll probably be selling one or two of my Dells in a couple months.