Decent City Building games on Steam.

Before I get yelled at, I have to say this. City Building games are my guilty pleasure, and I have played many. I have already beaten Tropico 4 on my buddies computer, and although ANNO 2070 looks like the most amazing citybuilding game I have seen in a long time, the cost of the game/ the fact that I won't be able to afford a graphics card that supports it for another 4 months is keeping me from purchasing it. So here is my question for you.

Does anyone have any recommendations as to a good City Building game? I have played many, and am looking for something new, that won't decimate my frame rate when I play it. I'm trying to stay in the ~$20 area due to budgeting. Sadly Dawn of Discovery looks interesting, but isn't available for some reason. Any opinions are appreciated.