I highly advise a PSU upgrade prior to any GPU upgrade.
You don't need a monster PSU but better quality is a must.
Corsair CMPSU-450VX 450W - $49.99 after rebatehttp://www.newegg.com/Product/Product.aspx?Item=N82E16817139003
This will ensure you are properly powering everything and give you the option to step up the GPU even more if you want.
I know the 9400 GT doesn't eat up much power but the rest of your set up is much higher end and you don't want a low brand PSU running near peak wattage all the time.