Someone I know had an old Dell Optiplex 330 minitower that he didn't need anymore so he gave it to me. I figured it would make a nice media server or something. It has a 2.66 GHz Core 2 Duo processor in it and while looking around for upgrades apparently you can hack the Socket 771 Xeon processors to work in a Socket 775 system with an adapter and cutting the alignment keys off the socket.
But the best part is the Xeon processors are 1/2 to 1/4 the cost of a similar Core 2 Quad processor. (Like $25 for the processor and $3 for the adapter)
Intel apparently swapped 2 pins on the Xeon so even if you defeated the keying mechanism on the socket it wouldn't work. So there are these little adapters made from some flat flex that slip under the processor that swap the pins back.
Interestingly the Optiplex 330 which seems to be the little brother of the Optiplex 755 (the 330 only has 2 memory slots) supports most of the Socket 771 Xeon processors in the E54 series, but the 755 doesn't seem to like them as much.
I looked at the performance of the E54xx series and they're pretty much inline with the first generation high end quad core i7 processors or dual core Haswell (generation 4) Core processors.
So if you have an older Core 2 Duo (or gasp LGA775 Celeron) based desktop laying around that needs a little processing help, this might be a cheap upgrade for you.