Yes it's compatible:
http://en.community.dell.com/support-forums/desktop/f/3514/t/19527428
Intel® Core™ 2 Quad Q9650/3.00GHz, 12M, 1333FSB
Intel® Core™ 2 Quad Q9550/2.83GHz, 12M, 1333FSB
Intel® Core™ 2 Quad Q9505/2.83GHz, 6M, 1333FSB
Intel® Core™ 2 Quad Q9400/2.66GHz, 6M, 1333FSB
Intel® Core™ 2 Quad Q8400/2.66GHz, 4M, 1333FSB