Just my $.02 worth here but if you're on a tight budget (something I can seriously relate to), depending on what you shoot, skip the kit lens for now. Unless you have a specific need for that 17-85 IS, that extra 11mm isn't really that big a deal for many people. I shoot a lot of landscapes and sunsets and while I have a Sigma 17-50mm, it's not very often that I have to break it out. And IS at these focal lengths...I never really saw the point. IS is great for long focal lengths where you really need a steady hand and a fast shutter speed, but I never really saw much advantage with wide angles. If you don't already have one, spend the difference on the cheapy tripod...you'll actually get more image stabilization that way than you will with the IS lens .
Honestly I shoot with a Tamron 28-80mm and a Tamron 70-300 on my Rebel XT and I get some really great shots with both and a 40D would do even better. Give or take on that extra 11mm, your probably not going to see too much of a performance difference from what you already have.
As was already stated, with the 40D you are going to have a 1.6x crop factor with the lenses you have but again depending on what you shoot, you may simply find that you'll be using your tripod more with the long end of the Tamron 75-300! LOL!!!
Seriously, -if- you really think you're going to need that extra 11mm based on your own shooting needs, then go for the kit but if not I'd save the money and just get the body. You can always get even better lenses down the road when you have more money and the two you have now will get you off to a good start.
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