I have a mac pro I inherited from a coworker, it came with two dual-core Woodcrest 3.0 GHz Xeons, 4 GB memory, etc. I recently upgraded it to a pair of 2.0 GHz clovertowns (more cores are more important to my work than clock speed), and I'm having, shall we say, mixed success. Booting into OS X, the system crawls along at several orders of magnitude slower than it should. The grey pinwheel progress bar at the gray apple logo screen on bootup pokes along at one tick every 7 seconds or so. When the login screen finally appears, the cursor blinks every 12-14 seconds, keystrokes take many many seconds to register. I never got past the log in screen. The same issues are experienced when booting from the OS X 10.5 install DVD. Here's where it gets interesting. My Vista 64 boot camp partition boots and runs just fine. I'm using it to post this message, in fact. It recognizes all 8 cores and everything. Has anyone here heard of such an experience?? Is there a way i can get into the EFI and get more information about what's going on, and why these chips are freaking out OS X, but Vista doesn't care?