450w is so much more than you need for 5670. you could have ran that on your 300w. if i were you go with a GTX 560 (non ti version). that requires a 450w PSU which is what you have. make sure you have 2 6 pin pci connectors. this card will run WOW on ultra. it runs battlefield 3 and skyrim on high for me!
if you dont have enough money go with a gtx 550 ti anything less than those cards isnt worth the upgrade iin my opinion. your cpu may bottlneck those cards though