Memory Upgrade

I have a Dell Studio 1555 running Windows 7 64bit. I am trying to upgrade the RAM memory from 4GB to 8GB (an upgrade that can be done on this model). The machine has two memory slots and two CPU (cores). When I boot up the computer recognizes that I have changed the memory, however it reports only 4GB (2GB for each CPU) rather than the expected 8GB (4GB for each CPU). In other words, no upgrade. The memory modules are correct - Two modules - 4GB PC2-6400 DDR2 - 800 MHz 200-Pin SO-DIMM Laptop Dell Studio 1555.


I have checked to see that the BIOS was current and chipset driver current and installed. I have also run a pre-boot memory test on the new modules.


Any ideas?