I have to agree with Jojo and others, the Sigma 50-500 is easily the best quality lens you will get in that price range. The Tamron 200-500 is not too shabby but it's about the same price as the 50-500. The 50-500 still beats it as it's faster on the focus slightly, and goes much wider also. The only thing IMO better about the Tamron 200-500 is it's lighter. Very hard to beat the quality and value of the Bigma, to get anything better or bigger you have to start talking some serious cash.
I love and drool over thoughts of owning the Sigmonster (theres your 500-800) but it's out of each of my budget limitations, if you can get passed the size/weight/cost issue of this beast it is beautiful. Prime like quality images, maybe the best images I've seen out of any telezoom come from this lens.
As for the Sigma 170-500, not something I'd put on my list. Slow on focus and at 500mm needs to be stopped down to at least f11. Not the sharpest of lenses, for it's price I don't think it's good value but then I'm sure some other people like it/ think it's ok. I did hear of someone that was happy with it, but they bought it secondhand for a fantastic price and didn't expect too much from it. Each to their own though, IMO I'd not buy it.
Anyways, thats my thoughs. I'm going to go back to dreaming and drooling over CDS and Liquids Sigmonster and their images.