Originally Posted by Tristicus
First off, I'm guessing you're using onboard graphics, else you would not be able to adjust the memory allocated to the display, correct?
4GB (4096MB) - 512MB = 3584MB
Now, if you take into account the usual deficit of "lost" memory of about 600MB used by other devices on a hardware-level, such as USB controller, LAN adapters, IDE/SATA controllers, etc., then the remaining memory is available to the system. In your case, 2815MB sounds about right.
You can perhaps increase the available system memory by decreasing the amount allocated to your on-board graphics. By installing a dedicated graphics card that uses it's own memory, you can reclaim the full 512MB back, and bring your system memory up to 3327MB, which is the usual for a 4GB system.
This is a hardware issue, and will remain so for as long as any attached devices (both on-board and add-on) use a 32-Bit memory addressing scheme. It has never been a problem before, because up until now, computers with 4GB of RAM where unheard of. Only when all hardware devices use 64-bit addressing on the hardware level will this issue go away.
Sorry I don't have any better news for you....