deadlyghost :
I just understood that your PSU does not have a 6pin connector that is required by the HD 7770... Correct me if I am wrong...
With a molex->pcie adapter this could be done,but i agree that a new psu is probably better,216watts on the +12voltrail isn't a lot.You have to take the power usage of the cpu also into account.
The corsair builder cx 430watt should already be sufficient,it has enough power and the required pcie connector.It also normally isn't the most expensive psu out there.