If you need to continue to support VB6 I would recommend creating a VM that contains XP and VB 6 with all the service packs on it. This way you can continue to run your development environment even though your desktop evolves to something that may be incompatible with the VB 6 dev environment. Installing Visual Studio 6 on Vista had issues two years ago.
For new development beyond maintenance I would look towards using a different environment. It's been my experience that you are better off looking at it from a completely fresh view point and not restrict yourself to migrating to VB .NET. It's enough of a hassle to migrate that you really should do new development in the best environment for your application. That may be VB .NET and it may not.
Developing using obsolete technology is never a problem until it's a problem and then it is too late. You need to stay in the sweet spot of the curve and you are the only one that can decide what that is. If you switch too early you will probably make the wrong decision and if you wait too long you will be too far behind. It's decisions like this that makes this field fun and painful at the same time.
