If I want to try XNA GS, is it just downlaod C# Express and XNS GS 2.0?

exactly ! you'll pay just if you want to port your game on XBOX.

another tool mae for xna
http://www.synapsegaming.com/sunburn_index.asp

loook amazing.

if you want ressources and tutorial www.ziggyware.com is the place to go.

microsoft provide some basic tutorial but prefered riemer's to begin.

don't hesitate to give some feed back.

I have some Java/MIDP 2.0 experience, I think C# and XNA combination looks straight forward to me. However I don't have time to try it yet, as my ordinary job draw all my time. But I will keep watching what happen in XNA, and see how Conitec respond to it