You should be fine with Vista Premium, but you are at Premium's max. Ultimate can handle up to 16GB of Ram though I doubt that you would ever need that much. With vista 8 to 12GB is fine for speeding up loading of programs which can be easilly loaded without having to setup a Usb Drive for Readyboost.
yeah 8gb will be more than enough. im running vista ultimate 64bit with 4gb of ram and thats plenty. if you where paying for the ram upgrade then i would advise against it but as its free there is no harm done