Hey there,
We are at 3.5. Best would be to start with that version. Then you would have to chose what language you want to use.
You have C# and VB.Net. C# is considered the faster language for the CLR to interpret, although it should make no difference in theorie. So you are more or less not bound by that. If you consider making huge .Net web applications I would go for C#.
To learn ASP.Net though VB might be easier to learn. There are plenty of converters and tutorials if you would ever like to switch to C#.
I would chose between the two by your current knowledge of programming/scripting. If you are used to VB through MS Office programming (vba), ASP programming I would definately go for VB.Net. If you currently know a language like PhP, java(script), C, C++ I would go for C#.Net.
Hope this helps
Yeroon