For Most games the i3 will be slightly better, but if you want to run things in the background go with the phenom.
It should no matter too much though, as most games are more graphics than cpu intensive, and both will run pretty much all the modern games with a good graphics card. I might wait for Llano or Bulldozer to come out; with Llano you would pretty much have a Radeon 5650 as your graphics card, which would handle most games (not crysis) on good settings.
A bulldozer 4 or 6 core would also be good , with its turbo core feature.