Hey, the differences between the express version and the full studio are that the full studio contains a lot more tools to aid your coding. The code you produce will compile the same, (so there's no limitations as per your output)... There's also limitations in the templates provided and inbuilt objects... But may i suggest
SharpDevelop, SharpDevelop is an Open Source IDE for .Net languages, which has the same full functionality of visual studio.
When i went down the VB.Net route myself(VB2005 at the time) i wen't down the Express Editions route, learned a fair amount, but then found sharp develop and found it to be (in my mind) a better IDE.
To answer your final question, yes Mastering express edition would allow you to work as a VB developer. But again using SharpDevelop allows you to branch out a little further =]
Cheers,
Jamey