Actually, mixing testing and unstable is more volatile and subject to breakage than just using pure unstable.
You just have to be a bit more attentive when installing packages if you use pure Sid (because packages may be "broken" in one way or another), but mixing Sarge and Sid can mean gcc and glibc conflicts that can break your whole system-- especially when dealing with a large group of packages like GNOME.
If I was you, I would head over to http://www.apt-get.org
and see if there was a backport to woody, of go to http://www.gnome.org
, download the source and compile it myself.
Hope this helps.