The sizing of the psu is determined by the graphics card/s. Without a discrete graphics card, you only need 250w or so.
One of the best budget psu's is the Corsair cx430:
At $25 after rebate, it is hard to beat:http://www.newegg.com/Product/Product.aspx?Item=N82E16817139026
Regardless, do not go cheap on quality in a psu.
Look for Corsair, XFX, Antec, PC P&C, or Seasonic of the appropriate size and you will not go wrong.