You should have very little problem migrating a 2000 DB up to 2005. Yes, there are lots of new things in SQL 2005, but ultimately, it's still a relational SQL database engine strongly based on the 2000 model.
For the user, going to ASP.NET doesn't even need to be noticeable. Of course, if you are going to upgrade a site to ASP.NET 2.0, you should take advantage of all that it has to offer (controls and usability enhancements, etc).
I would say the biggest technical challenge to this upgrade path is the different programming style from ASP to ASP.NET. You are going from an interpreted, non-object oriented, loosely typed, script style language to a compiled, OO, strongly typed, event based language and environment. It's a pretty big technological step.
Questions to ask yourself are:
What is my level of expertise in the target environment? Should I build some simple test applications in the new environment to get a feel for how it works so I can best determine how to migrate a legacy application?
A recent wrox book that I would recommend is ASP.NET 2.0 Instant Results
. It describes the construction of several applications from start to finish so you can get a good idea of how to build an ASP.NET app (versus just how to use control X and feature Y).
One thing is for sure: graduating from building apps in class ASP to .NET is a big step. It requires a whole different frame of mind. It would be very wise to just play around with .NET to get an idea of how it works before you start to migrate code to it. It's very easy to get yourself into the trap of bringing a whopping plate of spaghetti code to a .NET app and seriously detracting from .NET's capabilities.