Windows – Why can’t Windows 7 x64 use all installed memory

First of you will lose very much of your available memory with two graphic cards, that's 1GB of RAM lost right there plus the other devices.

There is a switch you can check in msconfig. Go to start, type msconfig and hit enter. Go to the boot tab, click advanced options. Check if the Max Memory option is enabled, if it is then disable it and reboot.

Edit:

The issue is with something called address space. On a 32-bit system you have 4GB of address space and all devices you need to use have to be mapped into the address space. Each device that gets mapped in will carve out a bit of the address space and render it unusable for the rest of the system.

The only way to get more than 2.5-3GB of usable memory in Windows is to install the 64-bit version. After that is installed it will look the same with only 2GB of memory usable. To get all available memory you need to go into BIOS and enable an option called memory remap.

The memory remap features moves all the mapped devices above the 4GB limit in the address space thus freeing up your address space for use by the system.

Turns out that the installed RAM was in an unsupported config -- On this machine, if you install 2GB chips, they have to be installed in triple, as referenced here (pdf).

Its interesting to note that Windows saw the memory, but it was rendered not usable and marked as "hardware reserved".