Portage will tell you:
You must rebuild all drivers if upgrading from xorg-server 1.5 or earlier, because the ABI changed. If you cannot start X because of module version mismatch errors, this is your problem.
You can generate a list of all installed packages in the x11-drivers category using this command:
emerge portage-utils; qlist -I -C x11-drivers/
Then after executing `qlist -I -C x11-drivers/', a list of x11 drivers package will show. RE-EMERGE THEM before startx, otherwise these drivers will not work. In my case, the keyboard and mouse stops responding and it takes me one hour to fix it.
Another weird thing is that when compiling xorg-server, I have to turn off the `hal' USE flag. Otherwise the mouse and keyboard will also stop responding. This is just my experience.
sth else: keep the USE flag of perl and libperl the same, otherwise when compiling some program, it will fail and say a long list of repeated complaints like unknown variable PERL_Gth..., whose exact term I can not recall. This happens to me while I was installing inkscape. emake failed
