W2k Server & Exchange 5.5

I am managing as subcontractor for a small company that is thinking of upgrading to Windows 2000 server. However, because of the hefty price tag involved (software, licencing and hardware upgrades), we would like to do it piecemeal. As a lready discovered, the Win NT4 PDC is the first to go.
However, I want to know if we can upgrade the standalone mail server running Windows NT4 and Exchange Server 5.5 to Windows 2000, and still use Exchange Server 5.5? Or will we HAVE to upgarde to Exchange 2000?
I have not been able to find any information on Microsoft’s websites relating to such a combination. Can anyone give me an answer?