depends on the game.
if it like multiple cores (crysis or GTA for instance) then the gains are massive.
if you can unlock the cores, then you pretty much have a 955 phenom ii, its the exact same thing. so for others, i wouldnt recomend upgrading until you try core unlocking (since you can potentially get the same performance).
i have a 550, but im waiting on a new MOBO woth ACC so i can try unlocking the other two cores. im hopefull, since mine is from the first batch. i know i guy on overclockers.net who unlocked his first batch 550 AND overclocked to 4.2ghz
its all about luck, but the performance gains are substantial so its worth trying.
in your case, id say upgrade to a 955 if ytou have the cash. a quad core is becoming more and more usefull for gaming. and by next year i think 99 percent of game will really see big gains with a 4 core proc.