In response to Dirk's question, I see the manufacturer's load capacity rating as a way of comparing tripods made by the same manufacturer for this purpose. For instance, if you're doing bird photography with a 35mm camera and a 600mm lens, it's going to be within load capacity of a Ser. 3 Gitzo, but bird photographers will often lug a Ser. 5 Gitzo or something similar for this purpose, because they need all the stability they can get with long lenses that magnify shutter vibration.

Of course there is the separate issue of tripods that have the same resonant frequency as the camera, which can happen in spite of high load capacity, and that just needs to be determined by testing.

I'd also concur with those who say the head is as important as the legset, if not more so.