Thr psu requirements are driven mostly by the graphics configuration.
Since your 9500GT does not need any auxiliary psu power leads, you should be fine with a 300w psu.
Whatever, buy only a quality psu. My short list would include Corsair, Seasonic, XFX, PC P&C, and Antec.
400w psu's are no more expensive, look at a Corsair cx430 which is dirt cheap at $25 after promotions:http://www.newegg.com/Product/Product.aspx?Item=N82E16817139026