I would go with a spread of lenses with some overlap vs. an all round general purpose from a wide angle to large zoom.
To cover a decent range the Sigma 17-70 is easy on the wallet and will provide sharp pics. Try to borrow a friends "favourite" lens and see how it performs in low light and how well it auto focuses. My Sigma 18-50mm f2.8 feels short for a walk around lens in a crop body camera. However I've taken great pics of my baby daughter. I do not regret buying my 18-50mm Sigma lens. If I purchased the sigma 18-200mm dont think it would have performed as well in my low light shots in my home. This horrible addiction has made me get a 24-70L lens knowing I take alot of pictures in my home.
After playing with a friends Kit lens (18-55) I was truly disgusted. At that point I knew how critical it became in strategically spending good money on lenses that will overlap in range.
Do your research and read up on reviews made by pro/amature/enthusiastic photographers rather than alot of technical graphs. The proof is in the pictures even though lab test are a good reference.
As I mentioned earlier.I'm on a steep learning curve aswell but I do alot research and choose my lenses wisely. For redundancy I may consider getting the 17-40L for the sake of more detail. This hobby is still cheaper than my drag racing addiction. The Sigma 17-70 seems to cover alot of ground with excellent results. Later on depending on enthusiasm you may go on the "L" bandwagon. For now I would find it hard to believe a newbie like myself disliking a Sigma 17-70 or 18-50 constant f2.8. Perhaps hardcore amateurs and pros will smirk at my comment but we must somewhere with decent glass.
