Haven't got my hands on a 975 yet but on the 965 and 955 I did.
Ok the 955 and 965 are almost the same chip. Well it appears to be the same chip but if you start tweaking both of them when you oc them you'll find the small amount of differences.
The 965 leaks more which means at the same clock as a 955 it will be hotter than the 955: The IMC AND the northbridge are a little bit better on the 965 aswell which results to better performances in benchmarks. You can use tighter timings with it.
But for overclocking on air the 955 would be the better choice as its a bit cooler than the 965. The 965 are more for beyond aircooling but it will be able to reach a higher clock than the 955 but not with conventional cooling