This page http://live.gnome.org/MemoryReduction/LowMemoryHacks describes some tricks/hacks to use gnome on a low memory machine.
I am running gnome on a 184mb Mac Powerbook G3. One of the things that helped me avoid swapping allot, but is missing from the website is replacing gnome-terminal by xterm, or urxvt. Gnome terminal is one of the biggest memory eater on my system. Another thing I do is setting metacity to reduced resource usage. (you can do this with gconf editor in /apps/metacity/general/), this often speeds up things like dragging windows around.
And I wouldn’t be me if I wouldn’t use this as a mpd/gmpc commercial. Mpd is low-resource and allows you to maintain a large music collection on low end hardware, I have ran mpd with 8000 songs on a 166mhz laptop with 28 mb ram.