ive got 3 lenses, sigma 24-70 f2.8, sigma 70-300 f4 and peleng 8mm fisheye. 24-70 is my walking around lens, but it is not an HSM lens, so the focus is slow for my taste when coupled with my 20d. i was not aware of the difference when i purchased it. now i want to buy used lens from ebay/craigslist to try out, a canon USM, to see if i can tell the difference and whether or not its worth it to me to sell the 24-70 i have now, and upgrade to something with a faster af. thoughts? canon 28-80, 28-105, and a canon 28-135 are what im considering, to use. they seem to be priced under 200 on ebay used and thats about where i want to be for a 'trial' lens. thoughts on this?