License questions

hey guys, Here's the situation. Exchange 5.5 died on winnt 4.0 machine. Boss wants to upgrade but keep costs down. We also have a W2k File Server with 30 CALS. He's thinking new Dell machine with another version of W2k running our old 5.5 (fighting against that one hard and need a few points to back me up.) My question is: Are server and exchange full versions only, or does MS also sell upgrade versions? Need this one really quick.

Thanks