For a system using a single GeForce GTS 240 graphics card NVIDIA specifies a minimum of a 450 Watt or greater power supply that has a combined +12 Volt continuous current rating of 24 Amps or greater and that has at least one 6-pin PCI Express supplementary power connector.
Depending on which brand and model of GeForce GTS 240 you get you may or may not need a 6-pin PCI Express supplementary power connector from the power supply unit.
Your current power supply will have no problems powering your system using the single GeForce GTS 240.