I am running Photoshop CS4 on a 2.8 GHz quad Mac Pro, 12 gigs ram and ATI X1900XT video card (Leopard 10.5.5). I am not experiencing any errors or slow-downs.
My thoughts are that it may be your Photoshop memory preferences and scratch disk allocation that are causing the problems. Ideally the scratch disk should be set to a separate drive as fast or faster than the OS drive (I use a separate internal drive for Time Machine back ups). It could also be a Time Machine bottleneck causing a system slow-down if set to an external drive.
Thanks Jerry. Time Machine is off, I enable it after I am finished for the night, so it is never running while I am working. I thought I set the scratch disk right, I have 2 separate disks in a RAID 0 that I use strictly for Photoshop scratch. I just reset prefs so I can't verify that, since it defaulted back to the main drive.
After reading Adobes guide I'm a little confused. I thought with 16gigs of available RAM I could set photoshop to use maximum RAM ... which is 3gigs. I assumed since I had so much overhead RAM it would work fine, even though Adobe recommends no more than 75%.
I had one crash last night after resetting prefs, fortunately it was when I quit the program after it finished saving the open files. If it continues I'll crank this down a little and see what happens. I remember reading a pdf on maximizing photoshop .. it talked about the memory settings. I'll find that and review it as well, may shed some light on what I"m doing wrong.