Frank,
Just upgrading to a new OS, as James said, will likely not give you better performance (maybe something I am looking past). Now upgrading to a better server, sure.
Now. There are only two things I am extremely passionate about on here:
1. Good backups
2. SMALL BUSINESS SERVER IS DEFINITELY THE WAY TO GO FOR A SERVER OS IF YOU ARE A SMALL BUSINESS. (Hence the term small business).
Micrososft has screwed us over since day one. The ONLY thing they have ever done that is benevolent besides Foundation Server is Small Business Server. I am not saying it out performs Windows Server 2008. It just gives you a lot more for less and is much easier to manage.
I would say the only reason you would want the regular Windows 2008 is if you are buying it for a bank or your hospital or some company that needs hundreds of clients (not 75) and needs to run multiple domains and sub-domains.
