SBS 2008 will provide a second server license so after a migration from SBS 2003 you can then setup the original server as a standard 2008 server for running SQL (that's actually how SBS 2008 is designed - SQL to run on the second box).
I wouldn't worry about staying with your current setup right now... but if there are performance issues, consider upgrading the hardware. Add RAM (unless you already have 4GB). Add a CPU (unless your board doesn't support two or you already have two). Really, the best thing to do is to review RAM utilization and add RAM (CPU is not normally a stressed component in an SBS server (it can be, but not often in my experience).