Installing Widows using just an upgrade disc?

I don't think so. You'll need a full version to install from scratch.

 

Because Vista is somewhat of a memory hog, we recommend to VMware Fusion users that they have at least 1 GB of memory assigned to Vista, minimum. More might be better.

But don't forget, your Mac OS X still needs RAM too. On my machine, a 15" MBP with a 2.4 chip, I have Leopard and 4gb of RAM. Although I mainly just use Win XP running under Fusion, I have to do demos with Vista and Linux running at the same time.

But RAM's not all that expensive if you get it from Crucial.com instead of Apple. Just remember to keep your stock RAM, because if you ever need support from Apple, they require Apple RAM in your machine to even look at it. So if you get 4gb from Crucial, don't sell your current 2gb on ebay or anything.

Your mileage may vary, but the users at the VMware Fusion forums are helpful in sharing best practices about this sort of thing: www.vmware.com/go/fusionforums