As you can tell from here:
http://technet.microsoft.com/en-us/library/aa996719.aspx under the Operating system requirement, Exchange 2010 doesn't run on any version of Windows 2003. so you'd have to upgrade to 2008 (which, to be honest, I highly recommend if it is possible for your budget).
That said, making sure that all of your operationally required software is fully updated is extremely important. Primarily for security, but also because of extra features, and the fact that SP3 is supposed to make Exchange 2007 not suck as much. And with *that* said, you should also approach it cautiously, making sure you have enough backup data and contingency plans in place should something go wrong.
