My system currently is using 8GB of RAM    Because of this , I have to install Vista Ultimate x64 in order to utilize the 8GB memory   

Windows Vista Edition 64-bit maximum memory support
Home Basic  ->  8 GB
Home Premium  ->  16 GB
Ultimate  ->  128+ GB
Business  ->  128+ GB
Enterprise  ->  128+ GB

But with Vista 64bit system, you would have software compatibility issues & problems....   Some of the program/software cannot run "NORMALly" as usual, and some can cause system hang. Example like Winzip (doesn't have 64bit yet), only can encrypt & decrypt, others advance functions cannot be used...