I've noticed this has been asked many, many times in the past, but I think my question/problem might be slightly different.
Over the last year I've found my Mac has slowed down to a point where it's virtually unusable to all but the simplest of tasks!
So, I did a fresh OS install on a new drive, which I'm running on now. Still the same problem!
On further investigation, I noticed this slowness is down to a memory problem (too much being allocated to the hard disk), and therefore the HD is working overtime to make up for a memory error.
My question is this; I have 5GB of memory under About This Mac/Memory, but only 1GB when profiling within Activity Monitor. Surely, this isn't correct? I know there is small discrepancies, but not this much. Surely there must be a memory error here, as in the computer is not using it's full quota of available memory. Instead, it's only using 1GB.
Please see the attached image, and notice the memory types and slot's I am using – 
Is there anything someone can help or spot here on the memory setup, or errors that would cause this either with hardware or software?
Best Answer
I would remove the two smaller DIMMs and see if the Activity Monitor sees 4GB, as it looks as if the 2 2GB DIMMs are not being seen. If this is the case, then keep the 512MBs out and upgrade the RAM, which is very inexpensive these days.
If you're planning to keep this Mac Pro for a while, its worth loading up on RAM even if you don't think you need it, because future OS version may require it. I've always tried to match RAM sizes in all slots, and not mix sizes, and even further, matching the DIMM manufacturer. This may just be an unneeded caution, but you should find out the exact RAM requirements for this particular Mac Pro on the Apple support site.