The 800 being closer to 1GHz is the best bet, although some of the lesser speeds will also do it.
You will have a higher percentage of 800's that oc to 1GHz than the others.
One other thing is that the date of manufacter for the 800's will be as good or better than all the others.
It's possible to get a 650, 700, 750 that was made months ago. The later the cpu is made the more time has passed for better revisions of cpu's. The 800's are relatively new to the market. So thier core may have been better optimized or refined.
I would think that a recent 750 would oc well also, if the place you purchase it will tell you the date of manufacter.