I am running gnome 2.30 from overlay. On my installation it is quite stable. In fact not got any errors so far in the month or so I have been using it.
I haven't tried xorg-server 1.8 series yet.
I would try one at the time. If you are already on xorg-server 1.7.6 (the latest stable) stay on that first. I installed Gnome 2.30
this way.
If, after that all is well and you still want to update to xorg 1.8, set the a global useflag: -hal. Then unmask xorg-server 1.8 and install it. You would probably want to do a world update after that (e.g. emerge -DNuva world), to rebuild anything that have used hal.
Good luck.
Mons