Going from 2GB to 4GB made a dramatic, noticeable difference to me.
My primary desktop (Windows XP) and my MBP are very similar from a CPU standpoint... They both have 2.4GHz Core2Duos, though the MBP has a slower bus speed. They also both have (or had, in the case of the MBP) 2GB of RAM, though the desktop has 4 512MB sticks of DDR400 and is running an i865 chipset (yes, you can run the C2D on an 865... if you know which board to get), while the MBP had 2 1GB sticks of DDR2-667, and is running an i965 chipset.
The MBP was ALWAYS slower than my desktop, to the point where I began considering selling or returning it. The most obvious, annoying issue was that typing in text boxes like this one was laggy to the point of dropping letters if an icon in the dock was bouncing. A fresh install of OSX did not cure this (not that this is surprising, the issue appeared the first day I owned the machine). Going to 4GB effectively eliminated this issue, and all of the other lags and slowdowns I was experiencing. I can now switch back and forth between the two without tearing my hair out while on the Mac.
For someone that is used to a slow machine, someone that doesn't have particularly high expectations of performance (read: someone who's never used a fresh, clean install of XP SP2 or 2k with nothing but the device drivers, AVG, Firefox and an IM client installed on a high end dual core/dual processor workstation), or someone who only uses their machine for basic stuff-not running as many as 20 or 30 Firefox tabs at once, plus Adium, Colloquy, Skype, Hardware Monitor, Photoshop and Winamp via Crossover like I do-2GB is sufficient.
If, on the other hand, you want the most performance you can get out of your SR MB/MBP, consider 4GB of RAM a necessity.