Dualcore will be faster in games, quadcore will be faster in SMP-tasks like rendering, compressing and other tasks which use multiple threads. Some new games appear to be slightly faster with quadcores and in the future this might increase, though Black Edition CPUs can possibly overclock higher which may be an argument too.
I would go for the Phenom, but neither choice would be bad.
------------------------------...man will occasionally stumble over the truth, but usually manages to pick himself up, walk over or around it, and carry on.
Reply to sub mesa
forget dual cores, even phenoms... they dont have a future... and even their present aplications are sparce... i advise buying the athlon x4 620, plenty of power for your money, but it still requires a bit of aplications ready for it... the most i can tell you is you can give a look at the athlon x3 435 at 2.9Ghz its a good all around cpu, but even so i still advise the x4 620, you can squeeze Hz out of a cpu with overclock if you ever need it, but getting another core is a lot harder =p