It's not so much about the cores the game uses, as the background ups.
If you are gaming while encoding while IMing while running anti-virus, and anti-spyware, the 6 cores come in handy.
Yeah but who in the real world actually does that? If you're encoding you'll want EVERY core at your disposal to do it. And how exactly do you IM while playing a game?
And I'm sorry, but if you have decent anti-malware software (i.e. not the McAfee and Norton commerical shite) then they don't take up enough system resources to warrant having their own core.
Personally, it's the Phenom II X4 955 BE for me - cheap, massive overclocking potential and 4 cores is enough for games. If you're on a budget get the Athlon II equivalent because the lack of L3 cache doesn't make any tangible difference.