If you're a gamer, with those choices, definitely Phenom II, it performs pretty much on-par with a C2D in a gaming situation.
C2D is still better if you do lots of heavy CPU stuff like encoding or photoshop filters or some such thing. For example my old E660 @ 3.0 ghz was able to complete SuperPI 1M faster than my Phenom II @ 3.5 ghz, from 17 seconds on the E6600 to just under 20 seconds on the X4 945... I estimate that I would need my processor at 4.1 ghz or so to beat it in that particular singlethreaded benchmark. But my 945 gets 17K in 3Dmark06 while my old system could barely hit 13.5K, using the same 4890. It is consistently better in the other gaming benchmarks that I've tested as well.
I'd recommend getting a X3 720 on the AM3 motherboard in my sig. If you just play games, you really don't need a quad core. Triple would do you just fine. It's almost a hundred bucks cheaper. With the extra cash, buy 2x2gb of DDR3 1333 mhz... it's like 50-80 bucks or so, depending on what's available. In the end you'll have a much better system, one which'll have a better upgrade path than you would with AM2+.
If you really want quad core, then just go right to the top, with the X4 955... there's literally nothing better from AMD right now (until the 965 that is) and it's as good as it gets from that side of the coin. It's only like 240 or so right now, sweet deal for a processor maker's top chip.