Some chipsets/BIOS only allocated 3.2 GB of RAM. I'm also using a Dell Inspiron E1705 and a Dell Precision M6300 with 2x2GB RAM, but the system can only use 3.2 GB because part of the RAM is allocated for hardware that needs memory address space.
A copy-paste from http://members.cox.net/slatteryt/RAM.html:
"Certain components within the computer require address space in the 4-GB range. Any address space reserved for these components cannot be used by computer memory.
The following components require memory address space:
- System ROM
- Integrated PCI devices, such as network connectors and SCSI controllers
- PCI cards
- Graphics card
- PCI Express cards (if applicable)
At start-up, the BIOS identifies the components that require address space. The BIOS dynamically calculates the amount of reserved address space required. The BIOS then subtracts the reserved address space from 4 GB to determine the amount of usable space.
If the total installed computer memory is less than the usable space, all installed computer memory is available for use only by the operating system."
If the total installed computer memory is equal to or greater than the usable address space, a small portion of installed memory is unavailable for use by the operating system."
Hopes this helps to understand. Sometimes it helps to enable PAE (Physical Address Extension), but not all motherboards/chipsets support this. Check Wikipedia for more detailed information.