Looks to me like one stick is only working partially.. only 518MB of it seems to be working, so the cause would point to the OS. If those 518MB were not being used, the cause would be Physical memory recognition, possibly RAM or mobo slot damage, but I would not yet discard the possibility till all the following checks are done: Windows checks:
1. Check in msconfig\boot\advanced\maximum memory http://support.gateway.com/s/software/microsof/vista/7515418/7515418su73.shtml
2. Instert the Windows disc and type SFC /SCANNOW on the Search bar, the Run command or the Command Prompt.
3. The last Windows option would be to do a repair installation or install a copy of Windows on a second partition. Physical RAM checks:
Do this to discard the possibility of RAM or mobo damage (see your system information after each check):
1. check if both RAM sticks are fully seated.
2. take both sticks out, clean the pins with a clean dry soft cloth giving it about five passes, install them back and try the computer.
3. if still nothing, try one stick at a time on each slot, checking the results
4. next reset the BIOS.
5. if still nothing, try your RAM in another computer and if possible also borrow a stick or two of working ram to try them on your computer so you can check for RAM slot damage.