I installed Debian 7 in the Dell Optplex 9020 Desktop with only one Monitor. But just cannot start gnome 3 instead in fallback mode (like gnome 2.5). I have installed the firmware-linux-nonfree xserver-xorg-video-intel xserver-xorg-video-ati. Thank you.
$lspci | grep "VGA" 00:02.0 VGA compatible controller: Intel Corporation Xeon E3-1200 v3/4th Gen Core Processor Integrated Graphics Controller (rev 06) 01:00.0 VGA compatible controller: Advanced Micro Devices, Inc. [AMD/ATI] Caicos XTX [Radeon HD 8490 / R5 235X OEM]
$xrandr ──(Thu,Oct02)─┘
xrandr: Failed to get size of gamma for output default
Screen 0: minimum 640 x 480, current 1680 x 1050, maximum 1680 x 1050
default connected 1680x1050+0+0 0mm x 0mm
1680x1050 0.0*
1280x1024 0.0
1024x768 0.0
800x600 0.0
640x480 0.0