for about the same price AFTER shipping
you can get a card thats literally a hundred times more powerful than the 8400
the 8400 is one of the worst cards EVER made it just plain sucks... avoid ALL 8 series cards and even 9 series (except the 9600 gt) that are below the 8800/9800 point...
a 300 w psu will probably be enough to run it... but i'd recommend getting a nice good 450 - 550 w one for cheap http://www.newegg.com/Product/Product.aspx?Item=N82E16817371006
I believe antec psu's are on the upper tier list of quality i think they are a tier 2 which isn't bad for this price
Although if you want to do any serious upgrading like high-end stuff a 430 w won't serve you at all