I personally wouldn't want to overclock my servers that much, for longevity reasons, I was only able to get my sandy bridge 2600k cpu up to 4.5ghz without having a voltage higher than 1.35v. It was running at 1.325v, but it wasn't a long term stable overclock as I needed to boost the voltage a little every month to keep that overclock, so I eventually went back to stock on that comp. The stock voltage for my 2600k is at 1.26v full clock idle, and 1.2v at full load. I never had a cooling problem with mine when I was overclocking the thing, it only got up to 60-65c most of the time, maybe 70c if it was warm in the room and I was running something that was maxing out the cpu.
The comps that I use for servers are my desktop and laptop, the desktop has a Intel core i7 2600k, 8gb of ram, and the laptop has a sandy bridge i3(Don't really remember the model number off of the top of my head.), also with 8gb of ram. Both comps are using 128GB SSD drives for their system drives, while the desktop has a few other drives for extra storage. I currently have my MC server on the laptop, as that one has linux on it, and the only servers that are on the desktop one are one's that only run on windows properly.
To mcmonkey's post, it was for cpu's running at 6+ ghz that needed liquid nitrogen to run.