a 650 or 7750 will be ok, those cards need little power (~50 watts or less)
but a 660 will need ~110 watts and a 6 pin power connection from the PSU; a 300 watt PSU won't have that nor is it a good idea to use an adapter for such low wattage.
bare minimum:CORSAIR Builder Series CX430 $44.99
$24.99 after $20.00 rebate card
better and allow ANY single graphics card in that build:CORSAIR Builder Series CX500 $59.99
$39.99 after $20.00 rebate card