One thing to keep in mind that using a different brand name will slow the focus down. This is what I have been told anyway. That on my D70 if I get a Sigma the focus on the camera would be slowed down. Never did a check so I trusted the guys word. More than just one person told me that. I have seen shots on the walls in that camera store that were done with a sigma telephoto as you describe and they looked very impressive. I was just there on the weekend so that was a topic of my discussion too.

"The happiness of a man in this life does not consist in the absence but in the mastery of his passions."