You get you what you pay for with the Canon EF 200-400mm f/4L IS USM Extender 1.4X: superlative performance and flexibility.
Versatile, sturdy, and built with high-quality optics, the Sigma APO 70-200mm F2.8 EX DG OS HSM offers a good value, although it's not exactly cheap.
The Canon EOS 60D is in many ways a great camera: fast, feature-packed, and with excellent photo and video quality. Some annoying aspects of its control layout dim its shine a little, however, so try before you buy.
The Nikon D300s is a great camera, especially if you need the burst speed or slightly improved low-light focus, but if you don't care about video you might consider looking for a really good deal on a D300 and using what you save to splurge on a good lens.