The people who keep saying there is a big difference are not really being very responsible.
CWA107's post put it well, and although the wording is complicated, if you only had 512MB or even 1GB, I'd say you would notice a performance increase, because anything lower than a gig, Leopard cannot perform optimally.
Essentially is really depends on what you're doing. If you're running applications that need more than 2 GBs of RAM, then having more RAM will maintain performance on the machine, as opposed to it slowing down due to lack of memory.
You should upgrade your RAM to 4GBs if:
- You're running a virtual machine and want to dedicate 768MBs or more to it (such as Vista on Parallels)
- You're doing video editing, especially if it's HD
- You run professional audio applications, such as Logic, and are using 32-bit 196khz audio tracks
If your typical session involves Safari, iTunes, stickies and an iLife app, you won't see any benefit in upgrading.
This is my machine right now, and this is a typical session for me. I have 2GBs of RAM, same as you.
I have 9 applications running, plus finder. I am barely using more than a GB of RAM. Firefox has 3 tabs open, iTunes is playing internet radio, MS Word is the biggest hog here using 49MBs, but even that is a 5 page document with citations and excel tables. If you're a real Apple purist, you'll be running Safari, Pages and Apple Mail, possibly shaving off a few MBs (although Safari has memory leaks).
The key figure on the screen shot is the Page Outs, which indicates Virtual Memory being paged to real memory. The page-in's are not so important.
Launching iPhoto (4,500 photos) and doing a search barely has any effect, in fact it grabs a mere 50MBs of RAM.
So, do you think you need more RAM?