Currently Being ModeratedSep 21, 2011 1:59 AM (in response to Max Ocklind)
Yes, you will be able to run Lion and if it runs in 64-bit mode is not that easy to answer, see here: Is Mac OS X Lion 64-bit only? How can I get it to run in 32-bit mode?
But usually this does not make any difference for the user as the apps that are designed to run in 64-bit mode by default (e.g. iTunes 10.4) will do so even on a 32-bit Kernel.
You could plug in 4 GB, but only 3 GB will be recognized.
But as you already have 2 GB installed, just buying an additional 1 GB module is about $14 on Amazon (see this list for other options). And if your RAM is 2 x 1 GB, I would go for buying a 2 GB module.
Currently Being ModeratedSep 21, 2011 2:33 AM (in response to probe_droid)
OK, thanks for the quick reply.
I currently have 2 x 1 GB RAM modules installed, and a friend of mine strongly recommended replacing them with 2 x 2 GB RAM modules, even if only 3 GB can be utilized.
I also found some more info here:
Currently Being ModeratedSep 21, 2011 2:47 AM (in response to Max Ocklind)
I cannot really tell if there is any difference in performance between having 1 x GB + 1 x 2 GB compared to 2 x 2 GB.
What I recommend is that if you buy RAM modules, buy some from a well known vendor and spend a few more bucks.
After I had to replace my 2 x 2 GB modules about a year ago, I bought Kingston ones (ones from other well known vendors will also do of course), and I realized that the boot time and the time to start apps was noticable faster than with the RAM modules that had been pre-installed.
Currently Being ModeratedMay 6, 2012 4:18 PM (in response to Max Ocklind)
A great explanation regarding the 3 GB limitation on everymac.com is:
The MacBook Pro Core 2 Duo presumably uses Intel's 945PM chipset, which can physically handle 4 GB of DDR2 RAM. However, a number of items must be stored in physical RAM space, and when RAM reaches 4 GB, there is some overlap.
In other words, in a 3 GB RAM configuration, there is no overlap with the memory ranges required for certain system functions. Between 3 GB and 4 GB, however, system memory attempts to occupy space that is already assigned to these functions. For instance, the PCI Express RAM allocation occurs at somewhere around 3.5 GB of RAM and requires 256 MB of RAM. Thus, the virtual space between 3.5 GB of RAM and 3.75 GB of RAM is occupied by PCI Express So in a system with 3 GB of RAM, nothing is being wasted because the memory space required by PCI Express is still between 3.5 and 3.75 GB, and the installed system RAM does not violate this space.
The net result is that at least 3 GB of RAM should be fully accessible, while when 4 GB of RAM installed, ~700 MB of of the RAM is overlapping critical system functions, making it non-addressable by the system.
Ultimately, 4 GB of RAM may be installed, but not all RAM in excess of 3 GB can be used due to the possibility of memory "overlap".