Well, if you have the money - I would very highly recommend this heater:
I have one in my 2 gallon tank and I couldn't be happier. Except for during water changes, the temperature doesn't waver at all. I'm very pleased with it! It's a little bit pricey, but you definitely get your moneys worth with this one.
A less expensive option is the Hydor Theo 25W adjustable one: http://www.drsfostersmith.com/produc...m?pcatid=11368
The thing with these heaters, that I've found, is that they aren't accurate. They work just fine - they just aren't accurate. I have one of mine set on 85F and it keeps the tank about 80F, another is at 83F and keeps the tank about 80F. So if you go with this option, I would put it in the tank without your fish for a few hours until you see how it's going to run. Set it at 79 to begin with and wait about an hour or two, then go check to see what the temperature is and adjust accordingly. Then wait another hour or so and go check again. Once it holds the steady temp you want, then add your fish back in :)
I hope that helps you :)