nvidia performance problems (again)

It’s that never ending story once again. Upgrade the video card, suffer extreme performance loss in the desktop:

1. swap nvidia 7900 card for 8600 (a meager upgrade, but still)
2. notice the desktop has dramatically slowed down (expected)
3. recompile the newest nvidia kernel drivers
4. reboot, and to no avail, no performance increase
5. repeat steps 3 and 4, each time tweaking something else. xorg.conf, ubuntu restricted drivers etc

DRI is enabled:
seb@hammerhead:~$ glxinfo | grep "direct"
direct rendering: Yes

I know glxgears is no good benchmark, but at least it provides a slight indication to the performance of the card. 5000 fps is a rubbish result. Also, I’d expect to be able to run the desktop with compositing without an problems - which I can’t.

Can anybody provide a fool proof method of fixing this? I’m reaching out for the sake of my sanity!