Deciding on a lens to improve my system

Hi everyone,

I was generously given some money earmarked especially for buying a lens. As such, I've been trying to think of what sort of lens would most improve what I can do with my camera. Right now I have the 17-85 IS, 70-300 IS and 50mm f/1.4.

I've been mostly leaning towards either the Sigma 50-500 OS or 150-500 OS, since I'd love to improve my ability to take wildlife. On the other hand, I'm intrigued by the quality of L lenses and have been looking at some of the more affordable L zooms.

Today, when I went to the camera store to look at the 150-500 again, the salesperson showed me a used 80-200 f/2.8L (made in 1991) that he recommended I consider. He suggested that plus a Canon 2x extender would still have excellent image quality and that without the extender the f/2.8L would be a clearly superior choice at a price similar to the Bigma OS. They want $950 for the f/2.8L, with 30-day warranty; it looks like it is in excellent condition.

Thoughts on which way I should go, between the lenses I mention above and perhaps something else I've overlooked? Should I even consider the used f/2.8L?

Thanks!

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Tim Butler, asisaid.com