The best value IMO is the 967MHz model with ACX cooler.
The performance doesn't go up much beyond 967MHz (base frequency) but the price does. Also, the performance is affected by TEMPERATURE so even if one is 3% faster theoretically on paper doesn't mean it will be in reality.
The FASTEST air-cooled 780 at 993MHz is 2.67% faster on paper but $60 more.