Yes, you'll definitely need a heater like
http://www.drsfostersmith.com/Produc...&N=2004+113767. We keep the fry at about 80-82.
http://www.fishlore.com/Forum/10-fis...-platys.0.html - here's a post we did on our platys. Check reply #13 and reply #15 for a lot of info that we posted to another fishlore member about setting up the adult tank for fry being dropped and setting up a tank for the fry to live in.
You'll know she's getting close when there are black dots in her stomach and the back end squares off.. but she still could be days from dropping.. they have a tendancy to surprise.
After you read those posts, feel free to ask any more questions you have. Good luck.. it's work and sadly a lot don't make it, but they are adorable, tiny & fun to watch.