The issue with those tests is that they really don't indicate anything as it relates to an individual user. Opening up a large image file, working on videos, running multiple VM's or mixing several audio tracks are all very easy ways to get up to, and exceed, the 2GB mark. These give us very little of the picture as it relates to our current personal usage. It's even less indicative of how programs might perform in the future.
What we really ought to be doing is looking at system performance after a reboot at some time interval of "normal" usage. Restart, use the computer normally for 1 or 2 hours (or even a typical 8-hr day), and see how much free RAM you have and how many page-outs are logged in the activity monitor. At least, that's what we used to recommend here at MacRumors. If you've worked your computer with a normal load, and you have relatively few page-outs, then you're currently fine as far as RAM is concerned.
I also want to point out this MR guide: http://guides.macrumors.com/Buying_RAM#Will_it_make_my_machine_faster.3F
I will say this, though, with 2GB minimum being the standard now, we don't have a lot of "do I need more RAM?" threads here, but I do think that a lot of people are upgrading to 4/6/8+ GB now because they can. And I do know that there are a lot of people here who legitimately upgrade because they need to, but there are a lot who have 4GB in a MB/P to surf, chat, listen to itunes and manage photos.
Personally, I find myself wavering between 2 and 4. Having 2 would be sufficient at this time, but I don't know how long I'll be keeping this computer, so I don't want to limit myself right off the bat as I did with my last computer.