That depends entirely on what you're doing with your computer. You may see no difference whatsoever.
I use photoshop and a few other RAM hungry applications. When I first got my MBP, i thought 1 gig was a little on the skimpy side.
With 4 gigs, I never question how much I'm using. I know when I have a lot of images open in PS, sometimes I get a little laggy, but that's unusual for me unless I'm doing stupid things and not really getting something done.