965 came out after 975. 965 has the ICH8(R) chipset, which allows for 10 SATA devices to be connected. 975 uses "older" ICH7(R) chipset. The 965 is considered the "weaker" board, but benchmarks point to something different as they are nearly on par.
965 is also the first target chipset for Conroe (Core 2 Duo), although newer revisions of the 975 support Conroe aswell.
Besides that, they are almost identical, except in the overclocking from what I've read.
I had a 975 which died on me, and I got a 965. Numbering aside, I believe it's a step up.