Well, you don't tell us what you are doing with VS 2002 or 2003, so it's
hard to say if you will see any improvement in going with VS 2005 but here
are some ideas:

1- Great improvement in speed for many aspects in .NET Framework 2.0.

2- Many of the new features of SQL-Server 2005 will be only accessible with
VS2005.

3- ASP.NET 2.0 is much more powerfull than 1.0.  For example, the DataView
is a great improvement on the DataGrid.

4- Idem for ADO.NET 2.0: better and faster than 1.0.

5- Finally, everyone will go with the .NET Framework 2.0 and VS2005.  I'm
not talking here about your clients but about designers of components such
as DevExpress, Infragistic, etc..  Don't expect them to add new features to
their old 2003 version of components after a while.

VS 2003 is now history and unless you want others to pass ahead of you, you
have to go with VS 2005.  This is particularly true if you plan to develop
with SQL-Server 2005.

1. Generics
2. Refactoring tools - best with C#
3. Improvements in speed for Framework
      even 1.1 assemblies run faster in 2.0
4. New sets of APIs - esp. good with ASP.NET
  a) Membership/Profile
  b) Login/Security
5. ASP.NET
  a) Master Pages
  b) Improved Data Grid
  c) Local web server so devs can develop on
     machines without IIS
6. Ability to have the server monitor changed data
      best with SQL 2005 and 2.0 Framework
7. Improved ADO.NET
  a) Faster
  b) More tools
  c) Declarative model (when needed)
  d) Better data access
8. Faster Development
  a) Better Intellisense
  b) IDE keeps up with developer
9. Partial classes (your idea)
      This is a dual edged sword, as the benefits
      come with some potential gotchas
10. Better debugging - much easier to find problems

If you include Team System, you get:

1. Class Designer
2. Unit Testing - similar to nAnt
3. Static Code Analysis

With the Architect and Team roles (different SKUs) or the Team Suite, you
get even more toys.