This boils down to a few things, but in general... The more RAM you have IF you're running multiple applications at the same time, the better. Realize however, that some apps are much better with handling resources than others. It's all in how well the apps code was written. Some apps are absolute resource hogs, while others tend to hover around an average for their type of app.
There's also the factor of RAM vs. processing power. Some apps rely more heavily on processing cycles more than they do RAM. If you have a steady group of apps that you use, and likely won't be switching from anytime soon, then you have to figure out which of those resources those apps are more likely to consume.
A great example: Aperture. It's a huge resource hog, but it tends to eat up tons more processing resources compared to RAM. So if you're using Aperture as your main DAM, then extra RAM might actually be a slight waste of time. It will help, but will that help be marginal or better ? That's the question.
The other question is.. are the apps you are running 64 bits? If not, then the increase in performance will also be less significant from what I've read.
And the main question right now is: Is everything laggy for you ? How do things run on your system in general ? Do you seriously need a boost in speed because you're working faster than your apps are able to respond to, or just because it would be cool to think that things will never slow down ?
I tend to run Lightroom and Photo Shop with Skype, Yahoo Messenger, iChat and FireFox at the same time, all the time. I have absolutely zero problems doing this on my 13" MacBook Pro with a 2.26 Ghz processor and 2 gigs of RAM. On occasion I'll get a 3-4 second beach ball when transitioning between things or if I do massive batch processing or import, but it's hardly worth mentioning.
Interestingly enough, I can't run ANYTHING along side Aperture when it's importing. Even when running it alone, it's far past the point of a beach ball. My system just becomes unresponsive, period. So if you're running into problems because of Aperture, I'd say upgrading RAM will do diddly squat. I've also tested Aperture on a 4 gig system, and it's the same garbage. Shame, too. Aperture seems a nice bit of kit when you're processing. Apple really need to get their arses in gear and work that out.
Doug