I have a nikon d80 and am looking to buy a telephoto lens to take sport photography. (eg surfing and rugby). I'v been looking at the Sigma AF 50-500mm f4-6.3 EX, is this a good choice, if not any advice?
thanks.

The problem with that lens is that the f/stops are going to kill you if you try to shoot inside or in poor lighting. It would prob be alright for surfing since I doubt many people want to go and surf in the evening moving into dusk but with rugby I think you would be out of luck.

For rugby I recommend the Nikon 80-200 F/2.8, I have one of the older models from what I have read the newer models weigh a little less and have two rings (one for zoom and one for focus, unlike mine which the one ring does both). It is really going to allow the light to come in. In fact it might be in your best interest to look into this lens along with a teleconverter it would most likely be cheaper to do it that way.

the 50-500 is not a good choice for sports. The 500mm is good for surfing, but as stated, the f/6.3 is SLOWWWW

I recommend something a bit longer than a 200mm for rugby, but it's expensive. 300mm f/2.8. That'll be nice for both surfing and rugby, because you will be able to isolate the subjects with the f/2.8 and be able to shoot low light or under lights. Throw a 2x on it and you have effectively a 600mm f/5.6 which is both longer and faster than the 50-500. That'll be good for shooting surfing. If you're not that serious about sports photography at this time then it might make sense to just go with a cheaper lens, as a 300 2.8 will run ~ 2500. It's not a cheap hobby.

As long and as fast as you can afford, is the usual advice that your going to get. I don't shoot surfing but I would think that the longer the better for it. I just got a 300 f4 that I've been coupling it with 1.4 extender and getting very good results for day light sports like baseball and track. It will be useless when night time football season get here though. So once again as long and as fast as you can afford.