Sigma 500mm f/4.5

I was in the same position as you are and I recently bought a Sigma 500mm f4.5. So far I really like it. It seems quite sharp especially close in. As I have not had it too long I am still learning. Looks like stopping down to f5.6 makes a noticeable improvement in sharpness. I don't know how this lens compares with the Nikon 500mm f4 but I like how it compares with my Nikon 200-400 vr1. I always thought the 200-400 was a bit soft at the long end and so I will probably sell it.

I see some are using the 300mm 2.8 with tc's and the photos posted look real good but my experience with tc's is that you always have trade-offs either from quality or in obtaining focus.

I was going to post a pic I put in my gallery but I can't get it to post here. If you look in my gallery the first pic in there is a female Baltimore Oriole. This pic was taken with Sigma 500 lens just the other day. Hope this helps.
kasjun