If you already have AC, let it run. Adding internal cooling to rack mounted server for only three days seems silly. If your ACs have trouble or you don't have AC (and you do have windows) then look for some cheap or used but working window units. Also, what kind of servers are these and what kind of OCing will you be doing? In my experience OEM servers don't offer any overclocking options and neither do many other boards made for real servers. If thats the case and you're relying on software to do it then you probably won't have a noticeable heat increase but maybe some instability due to not being able to up the voltage.