While 8GBs of RAM makes sense on a MBP with its limitation of two slots, upgrading an iMac to 8GBs by adding two 2GB modules doesn't. For a little extra you get two 4GB modules and when added to the 4GBs the iMac comes with, you get 12 GBs.
I have 12 GBs and I still get Page Outs in a small number, which indicates I do get RAM depletion. I have been tracking what Aperture does with RAM and I see spikes of usage. It would be a shame to buy 2GB modules and then find you really need more and have to discard them in favor of 4GB modules.
It is my expectation and hope that Lion will make even better use of RAM, so I expect that RAM demand will increase when I go to Lion. RAM is so darned cheap, it makes sense for the OS and apps to make heavy use of it. If by using more RAM they can offload I/Os and CPU usage and save me time, I'm all for it.
I have also been tracking Aperture's CPU usage on my i7 3.4GHz machine, and I see close to 400% CPU busy when I am reviewing images. Even with this CPU and my Aplib on SSD, it still takes about 1.5 seconds to finish rendering a RAW 18 megapixel image from a Canon 7D. Given the speed of the SSD, it seems most of that time is CPU time. Smaller images are of course much faster.
Do note that my comments were re: the MBP with 6750M and 1 GB video RAM, not the iMac with 6750M and 0.5 GB video RAM. I do not know how much the doubled amount of VRAM affects the performance I have observed. Edits of Nikon RAW files are instant.
I can not wrap my head around the proposed Library on MBP and Masters on another computer. This is antithetical to the single-user starting setup that I suggest. Maybe you have created a new Aperture method that will work fine, I guess you will find out.
On all machines viewing the Aperture library, click 'P' to enable Preview mode.
'P' on all machines for read-only access to the Aperture library
'P' off on all machines for read/write access to the Aperture library.
This will force the Aperture user into a Read-Only mode, safeguarding adjustments on masters.