I have 4 GB in mine. I thought that was enough, till I started working with VMs (Virtual machines). Wham, I'm out of memory in a flash. Now I wish I had 8GB for my VMs.
A typical arrangement: 1GB for OSX, 2GB for Windows Server 2008 in a VM (minimum required for it) and 1GB for Vista in another VM. (I'm studying setting up Server 2008 networks)
Admittedly it's a bit of a rare use but more and more people are working with VMs nowadays.