I have 4GB on a regular old Macbook and I've never had any trouble whatsoever keeping not only Xcode and Firefox running in the background, but also a VM runnin

I have 4GB on a regular old Macbook and I've never had any trouble whatsoever keeping not only Xcode and Firefox running in the background, but also a VM running XP, VS2008 and Google Chrome on it, among other programs. 8GB is useful if you need all of your (many many) programs ready to go at once or if you're dealing with HD video, though.