I've already RMA'd the first 1gb stick back to Crucial, and now I'm on the second-- same problems. Sometimes the system boots, sometimes it hangs at a blank screen until I hold the power button down long enough to turn it off. But when I CAN actually get up and running, "About This Mac" reports that I only have the onboard 512mb. System Profiler says my singular expansion slot as empty! Yes, I've tried re-seating it-- many times. To no avail. I'm also positive the module is as far into the slot as it can go. And I find it hard to believe that Crucial would send me two faulty sticks in a row. Unfortunately, due to my built-in 512mb, I can't boot off the Crucial ram alone. Any ideas?