It'll depend on the operating system you're using on your netbook.
1gb is a good amount for a Windows XP system, and there is a noticable improvement when going up from 512mb. Not so much when going from 1gb to 2gb. Anything more than that is probably a waste, especially considering what you're using a netbook for. It's not like you're gaming on it. Surfing and email and chatting does not tax a system, even a slow one like a netbook. Besides, 32-bit XP will not recognize and use much more than 3.5gb or so anyway.
Like XP above, Windows 7 Starter is also 32-bit and will not make good use of anything near 4gb, but it's still Windows 7 and it does like memory, so 2gb is a good amount.
So really, 2gb in a netbook is plenty unless you're doing something with it above and beyond what it's designed for. And you also have to take into account that your netbook may not take more than 2gb anyway.