I am running Vista 64 with 8GB.
It is nice. Yep, for sure.
Neccessary? Most likely not.
At startup, Vista 64 SP1 with a couple Window Sidebars and two programs auto-starting I am sucking up 1.1 GB of RAM.
Open a browser, add a program or two and I am easily at 1.9GB RAM consumption.
Start cranking the programs up and I hit 2.5 to 2.8 GB of RAM consumption.
(a good reason why 2GB is just not enough for smooth Vista operation)
The big hogs, photo and video editing, can get up over 4 GB. That is, of course, assuming everything is lleft open. The most I have every hit is 6GB.
Would I get 8GB? Yes, the cost of RAM is cheap enough. Better yet, I would buy 2x2GB and allow easy upgrade to 8GB if 8GB is really needed.