I regularly use more than 4GB of RAM with Ultimate 64, mainly by running firefox and a game (e.g. crysis or fallout). 8GB might be overkill, but with superfetch, its a nice thing to have and to never use a page file (hence mine is at 0) on the hard disk.
However I would go for the 4GB at that price, maybe even upgrade to 8GB if you have the money (that is soooo cheap compared to DDR3 or should that be DDR£?)