I shoot alot of wildlife and birding in the Everglades and nature reserve areas down here in Florida, and needed as much reach as I could get...though I am shooting with an Alpha A300, I bought a Tamron lens which might be worth a look if you really want serious distance with great IQ.
The Tamron 200-500 F5-6.3 is around $800 list, and you might find it cheaper shopping around. With your crop factor, you get over 700mm optically. Given reasonable light, it performs excellently, for birds, wildlife, and even action shots. CA/fringing is basically nonexistent, no distortion throughout the range, good, deep color, etc. It's a surprisingly good lens for the money, and built extremely ruggedly. It is also usable handheld, which is not easy for a lens that reaches this long (it's lighter and easier for me to handle than the 'Bigma' 50-500).
Some sample shots I've gotten from the lens:
A few 'action' type shots too, just to show some flexibility with the lens:
It wouldn't be an optimal lens choice for low light zoom work - but might be better optically than some average 200-300 lenses with extenders on them. If you want F2.8 at that distance, then you have to sell your car to pay for it - so it's a nice compromise that gives you superb range in good light and solid image quality compared to a cheap lens and extender combo - and all for $800 or less.
I've been happy with mine!