48 bit addressing...

hello everyone I have Dimension 4400 windows xp sp3, 1 gb ram, 160 gb hard drive(Seagate).  After installing xp on the new drive, I noticed I am only getting 137 gb of it.  127 actually after buffering 8 mb.  Is this system capable of 48 bit addressing?  I contacted support today and they told me 120 gb hard drive is the highest.  When I first installed Windows to the new drive I saw the hard drive as 160 gb at the install window.  I have checked out microsoft and they have ste[ by step instructions of how to fix this, however I want to be sure this will work and I won't be stuck with only 137gb hard drive.  Any feedback is appreciated?

Thanks...