I currently own a 450D and the prefered lens is a 70-200mm 4.5-5.6. Yesterday I shot my nieces first Rytmic Gymnastic competition and the outcome was rather poor. I think I neeed to upgrade so I can shoot better photos of this, although my main interest is probably taking photos when on vacation.There are several questions here. The 70-200 f/2.8L IS II is a wonderful lens that is sharp and renders very nicely. I highly recommend it. You do not need a new camera to enjoy its benefits. If you are shooting gymnastics, it will permit you to get a much faster shutter speed than your existing lens. The challenge will be rapid firing. The 450D will only shoot 3.5 frames per second. You may want to consider upgrading so that you can shoot more rapidly. The 7D would be a great upgrade in AF performance and firing speed.
I´ve been looking at the Canon Zoom EF 70-200 mm 2.8 IS II USM online and it looks really good, but from what I read it´s plain out silly to combine it with the camera I have? Do I also need a new body?
What SLR body and lense do you recommend for travelling (having in mind I´m a girl who gets tired of too heavy stuff) + the perfect small compact for when I don´t feel like taking the SLR with me. I´m so lost in the jungle of choices!!
The appropriate travelling lens is a tougher question. It depends on what you shoot. If you want to go simple, it is hard to go wrong with the 50mm f/1.4, which is fast and light. My walk around lens is the 24-105mm f/4L IS, which I recommend highly. It covers most of the focal lengths I usually shoot at. It's not light, but it's not heavy either, and it is lighter than the 24-70mm f/2.8L. However, f/4 is fairly slow, and so I always bring a fast lens like the 50mm f/1.4 which is a life saver in low light.
As for the possibility of a small, non-SLR camera, I have the Canon S95 point and shoot, which takes great shots and is tiny. It will fit in a small bag.