I have 4 gigs of RAM on my 2009 Toshiba laptop, which is plenty in every situation except for when I use Photoshop, Illustrator and any browser that is not IE simultaneously.
As for my desktop, I have 8 gigs, and I have never actually run out of RAM unless there has been a memory leak somewhere (I'm looking at you, Firefox) 8GB seems to be plenty for tons of tabs spread over 3 browsers, multiple large files in Photoshop and Illustrator. As well as any game I've tried. (although not gaming and browsers and Adobe stuff simultaneously of course) I don't really see the need to upgrade my RAM, although my MoBo apparently supports up to 64 GB