Why upgrade?
Well, for one thing, the Visual Studio .NET 2003 products and the .NET Framework 1.1 are about to reach their mainstream support end of life. . .
See http://support.microsoft.com/lifecycle/?p1=3040 and http://support.microsoft.com/lifecycle/?p1=1249 for more information.
There have been significant advances in the .NET world since 2003. If you haven't taken a look at Visual Studio 2008 and Visual Studio Team System 2008, you owe it to yourself to do so. You'll be glad you did.