2GB of RAM is fine for what you want to do. I'm constantly on CSS, and 2GB is fine. Used Vegas several times (can't say it's a hobby of mine), but it works perfectly. Never touched GIMP, so I can't say much on that. Unlike what most people say, you do not need more then 2GB for what you want.
But mind you, I've been using Vista for over 2 years, without any problems with 2GB of RAM (with, for the most part, only gaming). As I said above, I have never even come close to running out of memory, so why buy extra when you're not even going to use it?
And if you do find yourself out of memory, you can always buy more, as people suggested. RAM, in general, is quite cheap.