The 975X are "enthusiast" (no wonder the sky prices for them) driven chipsets. They have really strong C2D support and theyll support quad-cores. Theyre generally better at OCing too.
While the P965 is the "mainstream" C2D chipset, but many motherboards with this will support C2Q. But there are some (like the DS3 or P5B Deluxe) that overclock really well for its price range.