Well the P35 is the newest and arguably the successor to the 965 chipset, the 975 was the high-end Intel chipset, due to be replaced (shortly) with the X38 chipset.
The 975 essentially offered the wider feature set, and more bells, bangles and whistles, but the 965 was newer and leaner with a better Southbridge......and now the P35 is the most 'cutting-edge' chipset......until the X38 arrives
I guess, if you are buying today, the P35 is the variation of choice (not least because it will also support 45nm/Penryn etc. chips when they arrive).....and is available in DDR2 or DDR3 flavours