I would say 'Core 2 Duo E6700 (2.66GHz)' as looking at the spec sheet for the P965 chipset it only supports up to 1066mhz FSB. That said.....maybe if you can get a BIOS update from Dell you can stretch to a Core 2 Duo E6850 (3.00GHz,1333FSB). There's no mention of support for Quad core or Core2 Extreme <see below> so I think you'll be limited to the 65nm era C2Ds (could be wrong though)http://www.intel.com/cd/products/services/emea/eng/chipsets/288795.htm
All this said though...... I used to have this boardhttp://uk.asus.com/Motherboards/Intel_Socket_775/P5W_DH_Deluxe/#CPUS
which is S775 but the 975 chipset and with a bios update it was able to support pretty much any S775 cpu with some caveats. Then again Asus last sent out a BIOS update for that board in 2010 so it's whether Dell has been as nice about it's boards.
Hope this helps a little.