Upgrading from 3.5 to NT 4.0

I am currently running Exchange Server 4.0 on a Windows NT 3.51 box. I want to upgrade to NT server 4.0 and then exchange 5.5 , I am planning on upgrading to NT 4.0 first and then upgrading Exchange later. Is this a good or bad Idea, also what are the problems or issues I will incounter doing it in this way.