rule of thumb on changing cooler preference
 

Hi,
I am looking to improve my system but if it ain't broke , don't fix it.
I have a phenom x4 running at stock 3GHz. occasional OC. next year I want to move up to a faster processor.
My question is at what temperatures should I start about improving my cooling ?
At present I have an Akasa Evo AK-922 with 2 fans in push pull configuration and the MOBO controls the fans. in ambient I get an Idle temp of ~ 33deg C and when running hard with video , it goes up to ~ 50deg C.