If you're just using it as a server (which I'm assuming since you say you won't have a monitor for it), run a Unix. Anything you can get will run about 10 times faster than Windows. Plus, you'll be able to control it without having to hook a monitor back up to it. You can even get the console to run over the serial port so that you don't even need a video card at all (unless your motherboard requires one to boot or you need to change the BIOS in the future).

Plus you get to learn something new.

I'd recommend FreeBSD for this application, but Linux or OpenBSD would be okay. NetBSD would be a close third.