If you have a canister filter, I highly recommend the Hydor external inline heaters. I have a 300W on my 55 gallon, and it keeps my tank at exactly 80 degrees. They're a bit pricey (around $45.00 online), but I found mine on sale at a dinky LFS for $25.
The best part is that the heater is on the outside of the tank, so no ugly heater junking up the view!