I have this same setup....
My chiller is out for repairs right now so I am running with no chiller....My temp is about 81 which is a little bit higher than I would like...I believe most people on here shoot for lik 79.... Its winter right now so I can get away with no chiller...
In the summer I definately need a chiller on it....I bet the temp would get up to the mid 80s without it....
BTW~I have the current 1/10 hp chiller and I LOVE it....
Im about to buy the metal halides for my 24 gallon. IMO you shouldnt need to worry too much about heat issues if you have a small clamping fan blowing across the surface of the water.