I have tried going back and forth, it is reproducible EVERY time. Know what? If I take one of the sticks out (or swap in the old pair of 1GB sticks) I'll go back to a system that in certain, select cases can't keep up with my typing. Right now, of 4GB available, I have 2.4GB free. HOWEVER, it can certainly make a system more usable. It will also not remove the fact that the HD is (ignoring the optical drive) the single largest bottleneck in a computer in most cases. It will not make it equal the performance of one in any way, shape or form. No, going from 2GB to 4GB will not magically make your 2.4GHz C2D into a 3.0GHz C2D or whatever. Perhaps it would be more accurate to call it an increase in usability. You might also check out my RAM buyer's guide, specifically the first section titled 'How much ram do I need?' located here: I also found that the machine now feels about as responsive as an XP system with a virtually identical CPU on 2GB of RAM does with 2GB it felt extremely sluggish by comparison. The major difference I noticed going from 2GB to 4GB was that the machine no longer lagged when typing in text boxes such as this one while an application (such as Colloquy) bounced in the dock trying to get my attention. usually I have multiple FF windows open with 15 tabs or so each, and 10-15 IM conversations. For me, that's actually a pretty light load. Right now my MBP has Firefox open (with 6 tabs), Colloquy (in 4 channels on 2 servers), Adium (2 conversations), Skype, Hardware Monitor, Stickies, TextEdit, Calculator and Acrobat open.

It will depend on how you use the machine. I agree with brian, although I would say that it's more of a dramatic speed increase than it is a slight increase.

Macbook g4 ram cover