Activity Monitor reports incorrect amount of RAM

Question

I have an early '09 Mac Mini 2.0 Ghz. I bought a 2GB stick from crucial and installed it in the empty slot without a hitch, I was nervous, but ifixit was a great help. If I look at the "About this Mac" screen, it clearly now says 3GB. If I then click look under the system profiler, all is well there also. 1-1GB + 1-2GB for a total of 3. If I open Activity Monitor, under the pie chart where it is supposed to say the total amount of RAM installed on the machine, it says 2.75GB. What gives? This one really has me stumped. Strange thing is that if I add up the "free:" and "used:" from the column on the left, it adds to 3GB.
I check my other Macs, all 4 display the correct amount in activity monitor. 3 of them running Snow Leopard and 1 G4 running Leopard. I called Apple support and the guy I talked to was unsure, but said that I should call Crucial. Called them, and the guy told me he was CERTAIN that the RAM was fine if it was up and running without crashing, which it is. He said it would either not startup, or it would be crashing regularly. I downloaded a RAM testing software, recommended by the crucial guy, called rember. Ran it, and all tests passed.
A few months ago, shortly before installing the RAM, I was given an old G3 iBook. I wanted to put some software on it for my daughter and hooked it up via Target Disk Mode to my Mini. Soon after it was connected, I smelled an electrical fire. The Mini then shut down and the fan spun at max for over an hour, with no response to the power button and the light off. Eventually I unplugged it, then later plugged back in and restarted. It seemed to be working fine. When I tried to hook up a firewire HD, it wouldn't read it. Under system profiler it said "Unable to read Firewire Device" in red (first time I had ever seen color in the profiler!) letters. I took it to the genius bar and the eventually told me the logic board was bad. Week later got it back with a new LB. About a month later Snow Leopard came out and I then added the RAM and installed it. It had previously been running Leopard.
I know that somewhere along the line, there was an update specifically for '09 Mini's that said something to the effect of... to correct an issue with certain Mac Mini's recognizing certain 3rd party RAM. It was a EFI update!?! maybe!?! or some type of ROM update, I think. I installed that update and got the "Successfully installed" dialog box after restart. I am not certain that the "bad" logic board (TDM firewire issue) had anything to do with the RAM, or even if the issue was present before the new logic board.
Would a replacement logic board be the cause? Maybe I had done the ROM update on the Mini's old logic board, but the new one needs to be updated? Does anyone else have these issues with a Mini or even another computer? Or is it a bug in activity monitor?

Answer 1

You have to look at whether the specs state the GPU RAM is shared with main memory or not.
For example, for the latest Mac Minis, the spec states:
NVIDIA GeForce 9400M graphics processor with 128MB of DDR3 SDRAM shared with main memory
or
NVIDIA GeForce 9400M graphics processor with 256MB of DDR3 SDRAM shared with main memory
Compare to the high end iMac which simply states:
NVIDIA GeForce GT 130 with 512MB of GDDR3 memory
All current Macs use integrated GPUs that use shared main memory RAM for graphics save for iMacs with an NVIDIA GT or ATI Radeon GPU, MacBook Pros with the NVIDIA 9600M GPU, and Mac Pros.
On shared machines Snow Leopard is just more honest by noting that memory used by the GPU is not actually available for use by the OS.