And have a look at real results. If you intend on post processing images some of the examples in Flickr can be useful. If you check the Exif data it'll tell you what lens, camera, and settings they used and also what processing program.
I rarely shoot in jpeg. Lazy but it helps to tweak on a shot by shot basis. Lens wise have a good look at the groups on Flickr and see what is possible and also check a load of reviews. I still went with my sigma 10-20mm over the Tamron as both had pros n cons but the sigma seemed more friendly to my needs. Been reasonably happy with the results so far.