My experience is similar to another thread (http://forums.macrumors.com/showthread.php?t=433946), but I think it is different enough to merit a new thread. I purchased two sticks of 1gb RAM for my Early 2008 3.0 mac pro from Crucial. I installed them the way that Apple suggested: A1: Apple 1gb A2: Apple 1gb B1: Crucial 1gb B2: Crucial 1gb The computer did not recognize the RAM in the B riser, giving me only 2gb total. Under "Diagnostics" in the System Profiler, it said under "Power on Self-Test" that the RAM in riser B had failed. So I switched the risers with a similar result. It still only recognized the Apple RAM which was now in the B riser. I called Crucial. I was instructed to put the Crucial RAM in stock config: A1: Crucial 1gb B1: Crucial 1gb It worked so I said thanks and goodbye. However, I noticed that when I moved windows around, they moved in a very jerky way. It was obvious. The cause of this could be so many things that I didn't check... But I decided to put the Apple RAM back in the stock config and totally remove the Crucial RAM. It worked and the windows didn't jerk when moved. I ran an Xbench 1.3 test and saved it. And then I did the following config: A1: Apple 1gb A2: Crucial 1gb B1: Apple 1gb B2: Crucial 1gb This is not what Apple shows in the manual, but it worked. I had 4gb of RAM. And the windows moved without jerking. Xbench gave me faster results. I apologize for this being so long-winded, but I wanted to tell what I did as accurately as I could. Hopefully this explanation serves some useful purpose to someone. I do have some questions, though: 1. Does this sound right? 2. Should I be concerned about the Crucial memory? In the Apple System Profiler the Crucial RAM shows up as 800 MHz, so it seems as though it's the correct RAM. Thanks for listening.