eselect opengl set xorg-x11
emerge -C nvidia-drivers nvidia-settings
vi /etc/make.conf # set VIDEO="fglrx"
shutdown -h now
*you hear noises of a chassis being torn open, of screws being turned, a delighted "whoaah!" as the shiny red object is released from it's package, then another few screws turned and finally a loud *thud* as someone hit his head against the desk*
emerge ati-drivers # complained about kernel options
cd /usr/src/linux && make menuconfig && make install
emerge ati-drivers
eselect opengl set ati
emerge -avuDNt world # to honor the new USE flags
shutdown -r now
so what went wrong? Because - despite what everyone in here keeps saying - it just worked

Dual-Screen works, I can switch between VTs, I can spawn additional X servers, my composited desktop is stable, the wine games I used to play still work, ...
I was actually prepared to re-insert the old 7600GT if the drivers don't do, but no.. it works!

Granted, I had to re-apply some tweaks I made before, just differently:
- TwinView out, XRandR in. Works fine, I don't have to restart X to plug in the projector any more. \o/
- re-checked all of mplayer's video out plugins to see which one works best with the new driver. Just as with the 7600GT, VDPAU was unusable.
- apply a few different tweaks to wine games for a couple of fps. Too much shader-fu and they become CPU-bound, fglrx doesn't seem too strong on GLSL either.

Now it's not perfect, some 2D operations are slower (as expected), but kde3+fglrx is still faster than kde4+nvidia was. It works, I'm happy, and I can't wait to try the OS drivers in a couple of months (profiling applications is just so much more useful when every part of the system has debugging symbols).
Until then, I'll just gaze at the beauty of windows games above 800x600@low details.

weird enough, i noticed that portage carries 9.11 - isn't that supposed to NOT support the 5770? It still worked