From Dells support here http://tinyurl.com/dell64bit
 32-bit processors only have 32 address lines, limiting it to a memory addressing map of 4 gigabytes (GB).

The chipset, PCI devices, PCI-Express region, and any installed video cards use some of this map, and the BIOS correctly reports this range as unavailable to the operating system. If 4 GB of physical memory is installed, then the processor has no way to address the memory that overlaps with these regions that are already in use. The chipset directs memory access to the appropriate device rather than sending it to random access memory (RAM).

On Dell's newer lines of desktops, PCI-Express uses 500 megabytes (MB) in the map; integrated video takes 256 MB right below PCI-Express, leaving 3.25 GB of memory available to the operating system via RAM. In some cases, less RAM is available depending on what other add-in cards are installed.

http://support.microsoft.com/default.aspx?scid=kb;EN-US;q294418 see summary here for more info