The 17-70 is a great lens. And serious, IS on a lens like this is not needed IMO.
However, maybe if you want to sell it and get 2 lenses. The siggy 10-20 and the 24-70f2.8 to give you a really versatile kit.
10-20 - Extreme Wide Angle
24-70 f2.8 - Great low light and walk about lens (Although probably not wide enough for lanscapes for walkabout)
70-300 - Long voom.
Id look at investing in a nice 70-200 f2.8 next and then maybe a Tamron 17-50 f2.8 and you will have a great collection of low light lenses
Also, maybe consider a Nifty Fifty as they are peanuts!