Not much.
The system below, with your video card, uses under 300 watts under load and under 200 at idle. Your i5-2400 will use a bit less than the test system's i-7 920.
I'd get something in the 400 to 500 watt range unless you intend to get some other graphics card.
Processor: Intel Core i7 920(ES) @ 4.0Ghz (Turbo Mode Enabled)
Memory: Corsair 3x2GB Dominator DDR3 1600Mhz
Motherboard: Gigabyte EX58-UD5
Cooling: CoolIT Boreas mTEC + Scythe Fan Controller (Off for Power Consumption tests)
Disk Drive: Pioneer DVD Writer
Hard Drive: Western Digital Caviar Black 640GB
Power Supply: Corsair HX1000W
Monitor: Samsung 305T 30” widescreen LCD
OS: Windows 7 Ultimate N x64 SP1
NVIDIA GeForce GTX 560 1GB Review - Page 15