I'm currently running a DFI Infity P965 Dark-S Mainboard with Intel Core2Duo 6300 (1,86GHz, Conroe, OCed @ 2,8GHz).
What are my upgrade options for the CPU?
You'll find here on Intel's website the page describing the P965 chipset, including CPU compatibility.
The maximum speed of Front Side Bus (FSB) supported by this chipset is 1066 MHz. So the higher you can get is a Core 2 Quad Q6700 (8M Cache, 2.66 GHz) or Core 2 Extreme QX6800 (8M Cache, 2.93 GHz, probably too expensive for the limited gain of speed).
If you change CPU, make sure it has good enough cooling.