2GB ram is the max for the Core (non 2) line of Macs due to the chipset used. You can have 2x1gb sticks, or 1x2gb, but go above 2gb in some way and the machine will refuse to boot. I tried it on both a Core Mini, and Core MacBook Pro.

As for what is causing your slowdown issues in Lightroom, run Activity Monitor and check the System Memory, Disk Activity, and watch CPU usage of the process. That should help point out RAM, CPU or disk speed issues. If CPU usage never goes above 100%, Lightroom is not going beyond one core of CPU usage. For OS X, CPU time max is 100xNumber of CPUs. So 800% is a 8 way Mac Pro running all 8 cores at max.