For most people, I would recommend the 450D over the 40D. There are only a few things the 40D has that make it better. And most people won't really benefit from them. I have both and the ony reason I have a 40D is because sometimes I absolutely need the faster frame rate.

I think your budget is reasonable. I think the kit lens that comes with the 450D is pretty good. I recommend using it until you decide it's a problem. That way you can use more money for the long lens. And your money will count more there. I would also encourage you to consider buying a 70-200mm lens, instead of a 300mm. The Canon 70-200mm L lenses are really, really good - about as good as it gets. Affordable 300mm lenses are usually not so great. You will never regret buying the 70-200 f/4L IS, though. And you can always add a teleconverter later if you decide you really need the reach. Even with a teleconverter, the image quality will likely be better than most 70-300mm zooms.