I agree with everything paranoid android posted, but have a few thoughts to add.

> Yes, there is one application that is using
> a lot of memory (around 30%): Mozilla. I
> have no problems with my Gnome desktop.

There isn't any one part of Gnome that is a resource hog, but they do add up. I opened a Gnome session (I normally run KDE, which is a resource hog) and ran top. While no single component of Gnome was using more than 4% of total memory resources, everything that Gnome was running added up to about 15% of total memory. That's a lot, considering that a lightweight GUI such as Fluxbox probably won't use more than 1% or 2%. Of course, I could probably turn off some of the applets and such.

> I use Mozilla as a mail client and as a
> browser. If I wanted to use OpenOffice then
> I must close Mozilla first which is
> something that bothers me a lot.

That's just The Way It Is when you're running a system with limited memory. You shut down any unnecessary processes and run one application at a time. As p.a. mentioned, OpenOffice is a real resource hog. You might try the alternatives (Gnumeric for spreadsheets and Abiword for word processing) that he mentioned.

> I hope to
> overcome this problem with Gentoo. Also I
> would like to try SuSE.

I'm afraid that the applications will use about the same amount of memory regardless of what distro you are running. When I have more time I'll boot up some other systems and compare them, but I'll be surprised if I find any big differences.