I installed Windows 7 64-bit on a Dell Inspiron 530 with 4GB of RAM.  I noticed that under Computer > Properties, Windows was reporting 4GB of RAM but in parenthesis it said "(3.25GB Usable)".  Upon further research, I discovered that the motherboard is a FoxConn G33M02 which has a 64-bit chipset. 

The problem was resolved by running the latest BIOS update from Dell to correct the 64 bit memory addressing issues.

See the link below for the latest update as of this post:

http://support.dell.com/support/downloads/download.aspx?c=us&cs=22&l=en&s=dfh&releaseid=R212148&SystemID=INSP_DSKTP_D530&servicetag=CYV4LF1&os=WLH&osl=en&deviceid=14390&devlib=0&typecnt=0&vercnt=9&catid=-1&impid=-1&formatcnt=1&libid=1&typeid=-1&dateid=-1&formatid=-1&fileid=299050