Basically: 27, 49 and 75 Mhz are usable. If you buy a "real" RC car radio, it will be on 75Mhz.

27Mhz is full-power so you stand a chance of interference with A/C if you're within radio range (5 KM to be sure).
49Mhz is 100 milliwatts so the range is very short.
75Mhz is designated surface only (cars and boats).

Be aware of who is located in your surrouding area (5 KM) and you're off to the races