4GB of RAM in MacOSX 10.5, only 3GB in MacOSX 10.6

Solution 1:

I don't pretend to be an expert in this, but I think it's a more accurate relection of the usable RAM in 10.6, possibly as a result of RAM set aside for the video card?

This forum seems to address some users with your exact situation:

http://forums.macrumors.com/showthread.php?t=770235

Solution 2:

We just tested it again in MacOSX 10.5. The Activity Monitor shows up 4GB. However, free + reserved + active + inactive mem sums up to 3GB.

We also tried to allocate as much memory as possible. The maximum amount of used memory was 3GB. Interestingly, in the graphic stats diagram (saying "4GB") -- the green part saying how much free memory there is -- disappeared once we had used 3GB.

So it kind of seems that it is a bug in the diagram and it should have said "3GB" instead of "4GB". It probably doesn't calculate how much memory there really is in sum but just takes the value out of the system profile.