I have a 24" iMac with 3 GB of RAM.
When my computer freezes, it really freezes. No mouse, no sound (if I'm playing a file in iTunes, the music segment playing rapidly repeats). It seems the freezes occur when I have a number of applications open, especially video or processor intensive ones such as Starry Night Pro, Garage Band, Photoshop CS3, etc.
When I installed Leopard, I also installed the firmware "fix" for the freeze.
I have tried many things to pinpoint what is causing the freeze. The system and console logs are absolutely no help. I'm going to reinstall Leopard soon (as soon as SuperDuper is out in final form). I plan on erasing the HD, then bringing back my User Account etc. information once the Installation is complete.
AppleCare is absolutely no help re possible fixes.
Stay tuned. I'll post back when I do the reinstall (probably within a week).