For a start the 920 overclocks like mad so you dont need to waste the money on the 950. If your not overclocking though it is a faster stock chip but not worth the money really.
My advice to you would be to get a 5970 with a 920 DO and overclock the hell out of it.
My advice to you would be to get a 5970 with a 920 DO and overclock the hell out of it.