should i do it? i have the canon 18-55mm kit lens for my xsi currently. while it's a decently lens, i keep hearing great things about the tamron 17-50mm lens. i've read up on the reviews, as well as done a good bit of research here, but the lack of IS (or VC) tends to bug me just a little and considering i already HAVE a pretty decent EF-S 18-55mm lens, i want to ensure that my spending the additional $400+ is a worthwhile investment. obviously the fixed f/2.8 is a plus.
it would basically serve as my indoor/social gatherings lens, as my 50mm f1/8 prime works well, but the fixed 50mm can be cumbersome at times (just my opinion).
looking down the road, i envision pulling in a 70-200mm IS f/2.8L for my premier telephoto, a 100-400mm canon (although the "dust pump" nickname makes me nervous), a 100mm f/2.8 macro lens, and maybe a sigma 10-20mm lens. might i have all my bases covered at that point? 
anyway, my current focus seems to be on the tamron 17-50mm. with rumors about the 70-200mm IS f/2.8L being "obsoleted" though, that might play into my decision as well. gotta see.
any tips or recommendations on which path to go much appreciated. although the canon 17-55mm IS is a bit out of my price range for now. 

