It really depends on a number of things. As Kees points out, you can't use more than 4 GB unless you have a 64 bit version. In most cases 4 GB is all you need. However if you start a whole bunch of things and keep them open, you can run into paging which will slow you down. It also depends on what your're running. Some things take more memory than others. I've found that Chrome takes a good bit less RAM than IE9. Graphics applications, especially video editing, are known to be memory hogs.
I have a relatively fast laptop I used for about 10 months with 4 GB and it was fine. But I've gradually gotten where I start more things; when I got to about a dozen Internet tasks plus email, spider solitaire, and Wordpad, I'd occasionally run into pauses caused by paging. Since it only cost me $40 to upgrade to 8 GB, I did it, and now I don't get those pauses. YMMV