the Phenom IIs can be up to 20% faster Clock for Clock in games that utilize the L3 cache. they downclocked the Phenom to 2.6ghz to match the Athlon II. most of the time, the athlon was within 10% of the phenom, but when L3 cache was needed, the Phenom quickly pulled ahead to a 20% gain. the phenom will also OC further than the Athlon, furthering its lead.
the Phenom will reach higher clocks in the end, therefore substantially outperforming the Athlon II. i would try to get a Phenom if possible.
if your budget wont allow it, the Athlon II X4's are still great CPUs for $99!