In the past, I typically notice a speed bump in going from 2 to 4 gig since OSX needs to swap to disk a lot less with the 4gig. The more you run at once the more you'll notice it. Also programs like photoshop and vmware are very cpu and ram bound so they would benefit the most.
Aperture on the other hand is really more dependent on the GPU and while it could benefit from the extra ram, I don't think it would be as noticeable.