A brownout is when your voltage drops below the minimum that the receiver can operate on, which for Spektrum is 4 volts. You really can't know if your battery is up to the task until you hook up a meter to your receiver and put a load on a couple of your servos to mimic the current draw in flight. It's become common practice to use 6v packs with Spektrum equipment to handle the brownout issue.

The Turnigy/Frsky systems are NOT an upgrade from Spektrum. If you really want to get away from the Spektrum brand because you don't trust it anymore, Hitec and Airtronics have a very solid system that I haven't seen any reports of glitches from. Futaba doesn't suffer from the brownout issue but early units were sensitive to heat. In Maryland it wouldn't be an issue but some in the South had losses of control because of it.