Might want to grab a new power supply. Newegg has a 370 watt PC Power and Cooling unit for 45 I think, but with 27 amps, along with that grab a 4670 for your card, or maybe a 9600gso, should put you right around 100 bucks, give you a better card than the 9400gt, and not have to worry if your power supply will go out and take the computer with it....
Better yet, found this deal. Corsair makes a great PSU, 30 amps, so double the power basically of your current power supply, regular 50 bucks, 30 now after a mail in rebate.http://www.newegg.com/Product/Product.aspx?Item=N82E16817139008
For the video card, grab this perhaps. ASUS 9600 gso, regular 55, 35 after the rebate. http://www.newegg.com/Product/Product.aspx?Item=N82E16814121320
So if you get the rebates, then still under 70 bucks, and should destroy the 9400gt.