My LFS lost a whole batch of bettas when there store heater broke down on Sunday. By Monday morning, all his bettas had body fungus, none survived.
Anything under 74F will make them lethargic, any lower than 68F and they will likely get sick from the stress, lower than 56F will kill them very quickly.
Jazz, don't you have a 10gal? A 50-100watt heater would fit in there fine, set the heater on at 80F. Submersible heaters are generall better quality than 'hang-on' styles.
Inversely, over 84F constantly is not good (increases their metabolism/shortens life span) and over 95F will kill them quickly,
The biggest issue with water temperature however is drastic changes in water temperature, changing more than 2 degrees in a short period of time will stress any fish.
I have the digital (stick-on) thermometers for the tanks, and a floating glass thermometer to test the temperature for when I do water changes. If the change water is too cold, I put the bucket in a sink (I use bottled water no tap for now) and let it warm up.
More info than you asked for, but more is better!