Obviously the more you can get, the better. But i said CM500 in case you are a budget user. Dont worry as it still can bear upto a GTX260, GTX275, 4870, 4890, 5850 very easily.
IF you can (I said "IF") you can spend extra to get a better PSU, then i dont have any reason why you Should not do that :wink2:
... And no i havent seen any person running a 5770 on a CM500 because non of my known persons own any. However, a friend of mine is running an overclocked e8400 @ 3.8Ghz and with a GTX260 overclocked on a CM500 flawlessly, and its been more than 1.5 years its working just fine.