Most of what you carry, most of the time, if around the waist, does give you a real comfort and extra mobility so think about something like this one if that fits your body and lens attached. Ref is just to give the idea (You don't want one with a tight inner compartment).
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When on hand, a hand strap is the best aid you may call for. Pros with giant size camera like 1D and larger fast lenses use these straps all the time. The problem with those strap is your 40D or for that matter any body without built in battery grip, lacks one of the point to secure the strap so you need one that uses the cameras under surface tripod screw. I got the Olympus version from ebay that are going at shamefully low price but the problem even after cutting that extra ridge that is for Oly's body alignment securing, it still has the problem of covering the battery compartment door so I have to remove it every time I need to change the battery(And gush that happens too often and mostly when you really don't want it). Here is one cheap and right one for your EOS that fits and doesn't have that problem:
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As for the lens you know my first advise is wait for when you are finantially ready to step into one of the 4 sister L lenses in 70-200mm range. The non IS f/4 version is about $500 and with the expect to arrive soon canon's fall rebate you may put hand on one. You and your 40D deserve a better glass for sure. Of course you can argue the lack of extra reach, but sharpness and color render is unequivocal by any other canon lens or for that matter any other lens in the world. I would go with Sigma 100-300 for now that is a quit a reasonable lens.
Sigma 70-300 APO DG macro is possibly the best budget lenses in the range you are talking about (No IS).