How to choose server hardware for Exchange 2003 that can be effectively re-utilized for Exchange 12?

I disagree with this approach.  Technology changes so quickly, that by the  time Exchang 2007 comes out... your "upgrade" server will be out of date and it will probably be well suited for a paper weight.  Think about it, would you want to buy last year's technology or go with something on the cutting edge?  Take the 32bit vs 64 bit cpu's...64 bit cpu really have just came out this year ( meaning we have motherboards and software that use the full potential of the cpu ) are you going to stick with your 32 bit server or go with a cpu that is twice the speed?  What will you do when the 16 core cpu comes out in 2007?  

I think everyone should wait.  Use what you got now and upgrade when and if the software comes out.  Don't get caught in the hype.