I'm buying a barebones server. The motherboard has slots for two processors; do I have to use them both, or like RAM, can I add another CPU when I need it? I don't want to spend $500 on another CPU if I don't have to.
This depends on the exact motherboard and chipset, but generally for modern hardware that's similar to desktop architectures (e.g. Gulftown Xeons, which I'd guess you're looking at given the tags on the question and price point you mention) the answer is 'no, you don't have to'.
(If you could be more specific about the hardware you're looking at, this could be a firm answer.)