I'm be interested to know why everyone suddenly feels the need to stick 4GB of RAM in their MBPs?
Mac users seem to be getting as bad as PC fan-boys who get sucked into the Intel-Microsoft relationship who force their users to make unnecessary hardware upgrades.
Unless you're planning on using Vista/Windows 7 to run a load of seriously memory hungry/inefficient apps I can't see why you'd need more than 3GB tops.
In OSX you can be running Photoshop, Illustrator, iTunes, iPhoto, etc, etc all at the same time & it doesn't even use over 1GB (or 2GB if you've got some REALLY big graphics work going on).