What don't you like about the 17-50mm?
Your non-VC 17-50mm and the 28-75mm f/2.8 from Tamron are among the sharpest lenses you can buy for the money (or 3x the money for that matter), and the 24-105mm strikes me as an odd choice for a crop body. (That's my opinion, not to say it isn't the right choice for you)
I'm not sure how much of an "upgrade" (if at all) that would be speaking in terms of IQ...
What you do get are improvements in other areas: build, IS, weather sealing and a bit more telephoto (at the expense of wide) are these important to you?
There's a significant difference in perspective between the 17mm and 24mm, not to mention the 24-105mm is a slower lens.
If I were you I'd be keeping the 17-50mm and replace the 18-200mm with the 70-200mm f/4.0L IS if you're not happy with the performance of the Sigma.