I have a Canon 550D with the twin kit lenses. I would like to gradually upgrade and I have been looking at the Canon 70-200 4.0 L USM (with and without image stabilisation). Is the best way to go to get decent lenses and then upgrade the body? Will I notice the difference on a 550D body? What would be a good choice of lens combination? The IS lens seems the better choice but is heavier - so not so easy to carry around on travels. I don't have any particular interests - mainly travel / candid photos.