There aren't many differences between the 975X and P965 chipsets from a performance standpoint. The P965 officially supports DDR2-800 memory, but apart from several added features, its performance capabilities are very much the same as the 975X. If you read our evaluation of the 975X chipset, you will be familiar with Flex Memory and Matrix Storage Technology. This chipset officially doesn't support Memory Pipelining, but that doesn't seem to make a lot of a difference anyway.
The P965 is rated as a mainstream chipset instead of an enthusiast product. This, however, is mainly a positioning and marketing issue for the P965 top model. The result is that you won't find many real high-end motherboards based on the P965, but a lot of upper mainstream products instead. For the same reason, you also won't find many P965 motherboards with two x16 PCI Express slots for graphics.
The G965 comes with integrated graphics, the Q965 is the office version, and the Q963 a stripped-down version that doesn't allow adding a discrete PCI Express graphics card. A requirement to conquer homes and offices is Quiet System Technology (QST), which is a set of algorithms to intelligently control system and processor fans. The Q and P/G chipsets can be differentiated by Active Management technology for the office version, and Viiv Technology for the digital home.
One interesting difference from the 975X chipset is the more advanced ICH8 southbridge family. It offers six instead of four Serial ATA/300 ports with NCQ support and Matrix Storage Technology, and was designed with eSATA (external SATA) in mind.
You will find more information on the P965 chipset on the Intel website.