You're welcome Neil. I use Adobe CS5 extended and Adobe Camera Raw 6 as i always shoot RAW. Like any top software, its a bit of a minefield to start with, but eventually you get to grips with the main uses. But the indepth sub levels of CS5 is way beyond me. I doubt if i use more than about 10% of its complicated engine, but its enough for me.
The tweaks i used were very minor, as you have to be subtle, or you destroy things very quickly. If you havent got CS5 you would struggle to know what i'm talking about
What are you currently using for processing ?
AS for lenses, MF is fine for slow moving creatures, and landscapes / architecture. It gets harder with flighty / twitchy creatures and small birds. The AF 300 / F4 will be great in this respect. The IQ of that lens is well thought of, and can probably take a 1.4 x Teleconverter without much trouble, which gets you 420mm @ f5.6
To better the IQ of the Nikon 300/f4 in a sigma, you would probably need to go for the 120-300 f2.8 but these are big ££££££££. Its all relevant though - the sigma 120-300 has had great reviews using a 2x TC which gives 600mm @ f5.6, but you pay more