Canon 70-300 IS USM (
http://www.dpreview.com/news/0508/Canon/70-300-front.jpg ) Is a good lens for it's money. Image Stabilizer is a great thing. But the minus is, that at 300mm the picture gets quite soft. On the other hand, for it's money and at 200mm the picture is just great.
I'd personally go for 70-200L F4.0 USM, because it's not very expensive but it's still an L. Tho it lacks IS.
Those cheap sigmas are quite a good bang for the buck too. But i don't have any experience with those.
When you're buying a 70-300, you should probably buy some kind of replacement for the KIT too later on.
I at the moment, have a 400D with KIT lens too. Got to try 17-40L with it and ooohhh.... i just want it bad.:P