If you want to upgrade to Gnome 2.10, then the "testing" repositories has it. Since Sarge is frozen, you wouldn't get any further updates than the version it's frozen on.
Change your
/etc/apt/sources.list to point to the "testing" repositories and then do the following as root to update your whole system to the packages available in Etch (testing).
Code:
apt-get update
apt-get dist-upgrade