Well, i am running a Radeon HD 5970 (Basically XFire 5850) and i have a Phenom II X6 1090T OC @ 4GHz, And my PSU is only 850W (HX850W Corsair). I personally think that your right about getting a new PSU. So i would get the 850W, But your GPU's pull quite a bit of power so if in the future if you plan to chuck another one in then your just gonna have to upgrade again to the 900W
I would just go with the 850W, because i'm basically running the same GPU's as you and i have overclocked mine, and it works great.
Good to see another person with good taste in CPU's