I'm running a Mac Pro with 10.4.11 and 5 GB RAM. Never had problem with any applications even Photoshop. However, LR is a real memory hog. Just sorting images from a large shoot eats up all the RAM and then switches to VM. Quitting does not help, the memory is not released, need to reboot.
Is it common? Should I just buy more RAM?
I have a 24" iMac maxed out at 3 GB RAM ... LR library with 25,000 + images. No real problems. Never looked to see what it was using. Once in a while switching to PS or ID or other apps there is a pause to swap out memory (I'm guessing) but not to detrimental to the workflow .... but you know what they ... can't be too rich, too thin or have too much RAM
You need to check your "preferences". It shouldn't be running like that. I have 4GB ram and mine runs great. Uncheck auto updates and set catalogue backup to never and don't select 1 to 1 previews upon import. Those should help a lot.