my machine is a Dell Studio XPS 9100 with 8GB of RAM installed. When I open the BIOS setup and enter the System Info, it shows that my machine got 8GB of RAM installed but only 6GB is available (yes this is from the BIOS only). In Windows 7 (64bit Pro) it shows only 6GB of RAM are available. But if I run CPU-Z then it can detect all 8GB RAM. The machine came pre-configured and I have never opened the case before.
From what I can find about the now discontinued Studio 9100 is that it has two options for graphics, both of which appear to be integrated solutions. The numbers I can find show that it only has 512MB of memory for graphics, but it is possible that "upgraded" versions came with a larger chunk of dedicated video memory. 2GB seems like a lot but it is wholly possible since the graphics are integrated that 2GB of memory is dedicated to the graphics controller.
You can find out more about your card and exactly how much memory it is being allocated using the GPU-Z utility.