The Macs: Two 3.33GHz 6-core "Westmere" Mac Pros. 4 memory slots each. Memory: Each MP came with 3 x 1GB chips and an empty slot. Because mixing RAM is technically possible with this Mac Pro model, I ordered two 4GB chips from OWC with the intention of installing one in each for a total of 7GB in each machine. Result: No Go. If I put the 4GB chip into the 4th slot, the computer reads only the first two chips for a total of 2GB. Same result on both machines. If I put the 4GB chip in the first slot, it won't boot at all. If I put in one or both 4GB chips by themselves, the machine will boot just fine and recognize all of the RAM. OWC says: Must be a timing issue. Return the chips for replacement. My gut tells me that the same thing will happen with the replacement chips. So does anyone use this particular memory configuration successfully? Am I missing something here or should I do the RMA? Or should I order two more 4GB chips for 8GB in each machine and take the rebate on the 1GB chips? Any advice appreciated.