You can pick up an excellent condition used Nikon 80-200 2.8 AF-D for $700 (+/- $50). I've never used the Sigma so I can't comment there but I do own the Nikon 80-200 AF-D and I really like the lens. It's solid, fast focus, and produces great results. What camera body are you wanting to use it on?
I wonder what the AF-S version is selling for these days on the used market?