If anyone knows of any AMD cards that have the same performance as the GTX 480 and uses less wattage please give me a link, people have been telling me about the 7850 and how its cheaper...yet its like $50 more, some even $100.
if your current psu is 600w and a reliable brand, i see no reason to upgrade for a 480. you can plug all your system info into http://extreme.outervision.com/psucalculatorlite.jsp and click the calculate button and you should know the maximum wattage you will need. i always figure about 50 watts higher and your good as long as you have a reliable brand. Corsair, Seasonic, and Antec are pretty good brands.
The requirement of 600 watts is an estimate, as Nvidia doesnt know what PSU a person has or what hardware either. They tend to estimate a bit high. A GTX480 would run on a 550 watt unit if its good quality, and you dont have a ton of hard drives, lights, fans, and optical drives all running. Like the above posts say..if your psu is half decent, quality wise, and not like 10 years old, see if it works