Although more ram will always be helpful, it will probably be overkill with 4 gigs, as well as very very expensive.
If you want to see a more substantial increase in adobe suite speeds buy yourself a fast external hardrive with firewire 800.
divide the drive so you get a nice chunk of it that's completely clean and clear and never put anything into that part.
Use that part of the drive as your primary scratch disk for photoshop, illustrator, etc.
Watch your computer power soar!
Seriously, I use photoshop cs on my g4 1.2 ghz ibook at 8.5 x 11 at 300 dpi with usually around 20-30 layers.
I have 1.25 gigabytes of ram, but it's the external drive that really makes things buttery fast.
I get no slowdowns at almost any brush size. (ok, the really huge ones slow down, but that's not suprising.)