... you might want to consider these issues:
"EVERYTHING starts on the C: drive. You can move it later but it starts on C: so have a big C: drive."
"Also have LOTS of RAM. 4 GB is the minimum. The more the better. Obviously a good fast CPU(s) is (are) best."
"Seems the Microsoft migration takes quite a long time because it has to do a MOVE on each Exchange mailbox from SBS 2003 to 2008."
"It also changes the original SBS 2003 box so there is no roll back in case there is a 'problem'."
"The SBS MVPs attended a week long deep dive training on SBS 2008 just before the conference and the consensus was allow 4 days for the migration."
Of course, the cost of the migration is in addition to the cost of the new hardware and SBS Server Licences and Client Access Licences so even an entry level system for SBS 2008 is starting to look like a $20,000 investment.
So tell me again, why would you want to do this?