Umm... Ok here goes:
The Radeon's bottleneck is the memory not the core. If you increase the memory you may get a 5-10% boost in performance at 32bit running 800x600 or above. Increasing the core will do absolutely nothing, except disapate more heat. If you have increased your core speed and you "feel" that your games are running faster it must be a physicological thing because it actually does no such thing.
Leave your core at the default 166mhz, 183 if you have the VIVO. Bump your memory to 200 and adjust as needed until you recieve no artifacts in gameplay.