i do agree with everyone here that the 920 is way better in terms of value and performance, but to answer ur question, "what are the advantages of the 965 over the 920" is basically the flexibility. it's for the extreme enthusiast who's looking to squeeze the very last ounce of performance of their chip. with an unlocked multiplier, it gives u the advantage to try out different settings to reach ur goal of the highest overclock.
now, is it worth 5x the price of the 920? for most people no. logically, no. but if u don't care about price and don't care that you can only get about 200Mhz higher overclock than the 920, then by all means, go for it.
you'll probably get the most benefit of the 965 if you go to extreme cooling shooting for the overclock record. but if ur not shooting for the record and u don't have an extreme cooling set up, go for the 920, d0 stepping, and get the best value.
hope this helps