Update:

> I think I just found the problem! I decided to give the open source ATI
> drivers a try again, and replaced 'fglrx' by 'radeon' in my xorg.conf.
> And all is fast now. Wow. I had already forgotten how fast a desktop
> should be.
> 
> I'm not yet sure all is fine. On the first start, kwin crashed and all
> windows were on the first desktop, this never happened before. But it's
> working on the second login. I have no direct rendering, Xorg.0.log
> shows (EE) RADEON(0): [dri] RADEONDRIGetVersion failed (libdri too
> old). I had this before with xorg-server-1.7.7, now I'm emerging
> 1.8.2. Stay tuned.

I first got no dri with xorg-server 1.8, too, and then weird effects 
started happening, like the shift and ctrl keys not working. When tings 
crashed, I got a block screen and still could move the mouse, but that was 
all, even after Alt-SysRQ-R (which usually helps in these cases) I could 
not switch to a text terminal. But after a reboot all seemed perfect, but 
when I was writing a followe-up mail, X crashed. And I got several more 
crashes after working 5 minutes in KDE.

I downgraded back to xorg-server 1.7.7-r1, and now dri is working, too. 
Looking good! KDE is running for over an hour now, I'm not using any swap 
at all, amarok is emerging in the background, and mplayer is playing 
smoothly.

I experience little rendering problems in chromium (images appear over 
text) and quake3 feels a little sluggish, but I can live with this.

Thanks for anyone who helped!

	Wonko