What Operating System are you running on that new build machine?
Windows XP, Vista, Windows 7, Windows 8, each have a 32bit version and a 64bit version.
By design, 32bit version systems are only able to address a max of about 3.5gb which may appear as a different number depending upon your machine, but always less than 4gb.
If you want to be running with 4gb or more, you will necessarily have to be using a 64bit operating system.
note: While XP does have a 64bit version, it was not often used with home desktop systems and would probably be more problem than it is worth, therefore your principle options are Vista or Windows 7 in the 64bit version.