For a system
using a single GeForce 9400 GT
graphics card NVIDIA specifies a minimum of a 300 Watt or greater system power supply that has a maximum combined +12 Volt continuous current rating of 18 Amps or greater
Total Power Supply Wattage is NOT the crucial factor in power supply selection!!! Sufficient Total Combined Continuous Power/Current Available on the +12V Rail(s) rated at 45°C - 50°C ambient temperature, is the most critical factor.
Overclocking of the CPU and/or GPU(s) will require an additional increase to the maximum combined +12 Volt continuous current ratings, recommended above, to meet the increase in power required for the overclock. The additional amount required will depend on the magnitude of the overclock being attempted.
The Seasonic G-360 (SSR-360GP Active PFC F3)
power supply unit, with its maximum combined +12 Volt continuous current rating of 30 Amps
and with one (6+2)-pin PCI Express supplementary power connectors
, is way more than sufficient
to power your system configuration with a single GeForce 9400 GT