I bought my mac pro with the stock 1GB of memory, and bought another 2GB (2 x 1GB sticks) from OWC to upgrade it with. The extra memory just got here. I pulled out the second riser board, added the two extra RAM sticks on the back two slots, and booted my Mac up. And presto! Instead of 1GB, "About This Mac" now shows that I have... 2GB. Frankly, I had rather expected that taking a 1GB machine and adding an extra 2GB would yield a 3GB machine. Something is definitely fishy. I swapped the two memory riser boards. No change. I pulled out the original 1GB of memory and left just the 2GB I had added... and it worked fine, showing 2GB of total memory. I added the original 1GB back and it stubbornly remained at 2GB. I checked the System Profiler and it properly shows all four memory sticks, totalling 3GB of memory. But everywhere else in the system it says I only have 2GB. Can anyone explain just what the heck is going on here? Am I not allowed to mix-and-match memory sizes like this?