Not unless you have the habit of running a _LOT_ of apps at once. In effect, each app is gonna be limited to less than 4 gigs so with Photoshop running and needing 4 gigs and Bridge running–which uses ram differently and tops off at about 2 gigs, that would still leave about 3 gigs more or less free. However, if you are running other apps that are also ram hungry (Like Illustrator or InDesign or even Safari) adding a few more gigs could be beneficial...
If there is a LOT of free ram and you download and install ForceVM.plugin you can make use of ram beyond the 4 gig limit. See this story
Forve VM Buffering. So, for Photoshop CS3 only, there would be a benefit to adding more ram. I run a MacPro with 16 gigs of ram with the Force VM plug-in as well as having my PS scratch disk on a stripped array partition and Photoshop's pretty darn fast.
