This is a bit of a complicated issue. I'll do my best to keep it brief. Since you have a body that works, well stick with Digital Single Lens Reflex cameras that have interchangeable lenses.
There really is not a best camera for wildlife. I know youre talking about your situation, but wildlife images are made mostly with great lenses. Some "all in one" camers have good lenses but they fall short in other areas. . .
There are many ways to get good images including using the camera you now own -- lets stick with that. Lenses really are the key for the budget you are talking about. That camera can produce very nice images and 6mp is still good especially if youre only asking to make 11x14. If your output will be for high end magazines and huge enlargements, you may run into more of an issue, but I've sold covers shot with a d70 and the 80-400 vr - nikon of course. I've even got a cover shot taken with a 4 mp camera, so dont let MP get in your way. . .
My quick answer without getting into a bunch more why?s and what-fors is to tell you this:
Keep the camera and buy a good lens and tripod.
I think you could find a good, used 300 2.8 IS and a 1.4 extender and have yourself a great setup. There are other options, but I'm not sure spending money on a new camera at the expense of a new, better lens will get you better images.
As it is now, you cant, with that camera you own utilize it's full image potential with your current lenses. In other words, your not even using the camera you have to it's full potential.
Spend your money on a good 300 2.8 auto focus, or a good 100-400 or a good 300 f4, but don't spend a dime on cheap lenses and a new body, you will be going backwards if you do. . .
I hope this was at least marginally helpful. . .
Tony
www.tonybynum.com
"Roadless areas, in general, represent some of the best fish and wildlife habitat on public lands. The bad news is that there is nothing positive about a road where fish and wildlife habitat are concerned -- absolutely nothing." (B&C Professor, Jack Ward Thomas, Fair Chase, Fall 2005, p.10).