I know this is an old post but go with the gtx 550, I have the 430 gt currently and while it does play games the quality isn't that great. Sure it's cheaper but I'm already looking to upgrade from the 430 because games like SWTO chews up the card and craps it out even on lower settings. Your PSU should be able to handle the 550 as long as it is supplying enough power to the pci-e slot, mine is right at the minimum wattage and you cannot OC the card before you get video card crashes and bsod's.
So in closing I would grab the 550, I'd further say grab as much ram as your motherboard can handle and upgrade to win 7. You'll love it and your computer will respond a hella lot better!