I don't think any good graphics card will run that well on a 450w psu especially with skyrim at mid to high settings. Im probably not the best person to take advise from but i would probably save up for a new power supply with a minimum wattage of 550 that is made buy a trusted manufacture such as Corsair, Antec or Enermax and i would probably buy an hd7770 or maybe even an hd6870.