The max amount of memory that the 32 bit version can support is 4 GIG.  Even if you did have four GIG the most that will be reported in system manager will be 3.12 GIG. Below is a link to the Microsoft site talking about what causes this.  This will be solved in Vista SP1 when it is released sometime in March 2008.

Microsoft KB Article For Memory Usage
http://support.microsoft.com/kb/929605