Doug,
keep in mind that you can't mount extension tubes or telextenders to the 17-55mm F2.8IS. The rear element protrudes too far back. So, if you want to use the extension tubes for macro work, you really should pick up one of the primes and -- for me -- the longer the better for macro work. When I'm doing extreme close-ups with the 200 and max extension tubes, I'm working within 2 to 3 feet of the subject. I'm just sayin'! My next lens purchase will probably be the 135mm F2.0L and I'm betting it will really rock for macro work with tubes.

Jim