Personally, I am very happy with VMWare Fusion on my Macbook Pro. I can comfortably run OS X and other platforms running simultaneously, in my case: Ubuntu, Debian and Windows XP.
4 GB of RAM isn't strictly necessary but makes things more comfortable. I fully agree with jballanc about Spaces and full screen mode - that is really nice!
Consider also that VMWare has good support for virtual networking. For instance, you can have your Debian server running in one virtual machine serving desktop clients running in their own virtual machines. Testing client/server stuff that way has saved me lots of time.