IMHO, while Microsoft doesn't seem to want to own, that DWM (Desktop Window Manager) has resource issues, it's been my experience that the service, essential to the Aero-glass appearance on Windows 7, can easily be "maxed out" if there is too much pixel real estate in front of you. My current environment uses two dedicated graphics adapters (AMD HD 5450 and AMD HD 6450) for 3 1600x1200 monitors. And the Aero-glass appearance can only be maintained for a short time before the system automatically shifts to "Basic" appearance mode (and a varienty of Event ID errors, 9024/9025/9026, are logged in the Application log). Restarting the DWM service re-enables the Aero-glass mode, but that will only last a short time.
You can get an idea of the resources being exhausted through Microsoft's SysInternal's Process Explorer tool; a tool which provides far greater details about each Window's process and is available as a free download from technet.microsoft.com/en-us/sysinternals/bb896653. Once running this tool, select the dwm.exe process, right-click for Properties, then select the GPU Graph option. You will see how close to maximum the resources are for this service.