I don't have experience with overclocking servers, but it stands to reasons that you would have the same issues that I've seen from overclocking desktop PCs. The most obvious downside is that you may damage your CPU. Increased heat is going to result from overclocking, and you will have to address it, whether through heatsinks, or increased ventilation or both. If you are experiencing any heat issues currently, they will be magnified when you overclock. Another common issue I experienced was lack of stability with high system bus speeds. I faced all too many BSODs playing around with overclocking, although you can sometimes regain some stability by altering the refresh rate of memory from BIOS. I've also burned chips by pushing the stock voltage rating too far - generally 0.3 volts over stock is as far as I could go without ending up with a burned chip.
Most of the problems I've experience over the years have probably been from pushing overclocking too far, but generally I was doing it with borderline obsolete equipment that was expendable. Done carefully and conservatively, I'm sure that the results would be more positive. But even so, you are always taking a bit of a risk with overclocking.
I would proceed with caution if you decide to overclock. It may affect the warranty and/or life of some of your computer's components. It may also generate significant extra heat.
What exactly would you hope to get out of overclocking a server? I doubt the performance difference would be worth the risk. Perhaps buying a new server might work better? Different software? Upgrading your server's components?