PD7 and Windows 7 64 bit

I have one machine with PD7 and Windows 7 that allows the ram to be set at 8GB with a green bar on the memory configuration screen. On a second machine the bar is green only to the 4GB area with the rest of it up to 8GB saying not recommended.

Why can't I set 8GB on both machines?

Both installs are PD7 and Windows 64 bit.