Hey Ingrid,
It's me, Jeff! First post here.
If you want more information on lenses, go to fredmiranda.com . The "reviews" section there has user-based reviews on many lenses. Once you find one that looks promising, go to the forums and do a search and see what others have commented on and maybe find a few samples posted.
Another way to improve your photography is to switch to Canon (that was an ongoing joke intended just for Ingrid so please don't flame me).
I have and use the Sigma 24-70mm f2.8. When I first got it I found it to be soft so I sent it back to Sigma for calibration and/or to be rechipped. I now am very satisfied with it. Many others have found it to be just as sharp and fast AF as the Canon 24-70 f2.8 'L'.
Have Fun!
Jeff
Zingpix