Also have that MB (w/E6400 OCed to 3.2 GHz.). For a 965 MB it is very good.
For DX-11, it is not about the MB. DX-11 is dependent on GPU card and Operating System (DX-11 I believe is only suported in Windows 7.
As to GPU and MB, Looking at reviews on newegg there were several running on older MB (using Q6600, some using older MBs than the 775.
Can not remeber if that P965-DQ6 was PCI-e ver 1, or 2.0. Most new GPUs are ver 2.0 or 2.1 with most of them backwards compatable to pci-e 1.0.