Hmm, I'm not so sure the lockups are KDE related but I intend to find out when I get home tonight. I've experienced the same thing on one of my systems running kubuntu 8.04. It happens with any 3D game and sometimes when watching DVD's. From what I had read, the issues stem from my XFX 790i ultra board. My older system has an ASUS M2N32-SLI and it runs just fine.

It's been my understanding that there are issues with the firmware in the nforce 7x0i series boards and some combination of geforce driver and kernel 2.6.24 and older. I'm upgrading the newer system to Kubuntu 8.10 beta this week to see if the newer kernel doesn't resolve the lockups. That system is stable when running XP. I'll give gnome a try tonight to see if that makes a difference.

-twig