My 1st gen dual core (MacPro1,1) with 16GB (4x4gb) has served me well for the last 5 years but it's now time to retire it. I just bought a used quad-core (MacPro3,1) for a really great price but the previous owner kept all of his RAM, so I installed the 16GB kit from my old Mac Pro. I didn't realize that Apple specifications call for PC2-6400f in the MacPro3,1 and I've been running it with 5300 for the last 3 days without any problems. Is this bad? Am I bottle-necking my machine? I want to max it out with 32GB, but my dilemma is dollars. I would prefer to keep my existing RAM and just buy the other 16GB but not if it's going to slow the computer down.