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)
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.
Thanks for that input. From the little I've looked so far I was thinking maybe 2.6 would be close to as high as maybe could get and you solidified that idea so far. I did update my bios couple weeks ago but it's a couple years old itself. Guess I'll try to find some bios notes hopefully and find out if can look into something higher.