Check out the downloads section of this forum for starters. There are names of some good old subsims that you might find interesting.
http://www.subsim.com/radioroom/downloads.php
Aces of the Deep, SH2, SH1, Shells of Fury, Jane's 688(I), etc. These are all classics and should run smoothly on your system. Check em out!

I think you might be able to purchase them on Steam or Amazon.
I know what you mean mate, netbooks are awesome as long as you don't plan on running (moderately) high end games on them. I almost went for one but then decided I couldn't do without a couple of games in my laptop. Am very satisfied playing SH3 on my new Lenovo.