I too would avoid the 50D. Pick up a used 40D instead, or even an old 5D. Only get a 7D if you want the video features and 18MP (even the new T2i would be fine for that though). You won't notice much of an improvement from camera to camera as much as you would a new lens. You can always buy a new body and sell your old one, but the lenses are what you keep for years and years.
As previously mentioned, the 24-70 f/2.8L is a fantastic lens for every day use. That lens is on my camera the majority of the time when I visit the parks. If you can't see yourself upgrading to full frame in the future, then you can get the 17-55 f/2.8 IS, but I have heard it has a cheaper build quality than the L lenses and for $1,000 you want something that'll last. If I am going in the evening/night or want to travel light, I generally bring my 50 f/1.4.
Keep in mind that the 24-70 is a large and heavy lens, and when put it on a Rebel body (like the XTi) it will not be too comfortable. You can invest in a battery grip to help offset the weight and provide a larger area to grip. If you want a bigger lens, getting a grip is very beneficial.
