You need to know C#, but you don't need to be an expert with the language to begin writing games. With your background in VB.NET picking up C# should prove relatively easy for you. The IDE is the same and the languages work similarly. Just need to learn the syntax and you're good to go. I would recommend reading through the information
in this FAQ for some resource to get you started with learning C#.
Once you have a pretty good grasp on the syntax, you can browse around through the
links in this FAQ to find some good resources for getting started with XNA. (there's a lot of other good information about XNA in the
FAQ forum just in general that you might want to browse through).
There are a few books out there for XNA game development, but at this point, they are going to be a little behind the times. Most were written for the XNA 1.0 framework and with the release of the XNA 2.0 framework just recently, you're going to find some differences. They still have a lot of other great information and material in them, but you may have to do some extra work to get projects and examples they are showing to work with the most recent release of the framework.
Hope that helps get you started and good luck!