350w would be the bare minimum from a quality psu, cheap psu's of that wattage usually fail and take out other components, Your voltage drop with the gpu installed says that your current 300W unit is living on a fine edge. check out the...
http://www.newegg.com/Product/Produc...82E16817371005
http://www.newegg.com/Product/Produc...82E16817371006