I have a 20-30 G UTH on my 40G tank, and it works wonders, but then again my house stays at a pretty good temp (not cold, not hot, like right in the middle ) But like mrshawt said, Ive noticed if your house/room temps are too hot or too cold, a smaller/bigger UTH is probably needed than what you originally planned.

And the Lutron dimmer she recommended is awesoommeeee

So go with that, its like 11 something after taxes. The UTH I have I think was 40 bucks after tax? So a 30-40 might be a bit more than that.