My wife is friends with a home-schooling family that is looking for a bargain machine that they can use for surfing and for their home-schooling software. Their limit is probably $350. I came across this used HP 6000 Pro on eBay.
I have a Gigabyte motherboard and cpu I inherited from my son when he bought a new gaming machine. I changed the BIOS from 8x to 6x multiplier as I don't need the speed and I'd rather it ran cooler and maybe more reliably. However, in system settings > details I don't think the reported speed changed.
I overclocked my Intel Core i5 3570K to 4.2GHz at 1.1V. It ran stably overnight on Prime95, but, while I was monitoring the core temps, they stuck within 45-55C with core #3 was always 3 or 4 degrees higher than all the other cores. Then after 11:50 Hours I stopped the workers and saw that all cores completed around 810, except core 3 which did 707 tests.