im guessing youre talking about the AMD phenom ii x4 965 because its in the same price range as the i3-2100. If you overclock the phenom to 3.7 ghz, you average about 6 FPS less than the i3-2100 for starcraft 2 on maximum settings at 1920 x 1080.http://www.tomshardware.com/reviews/gaming-fx-pentium-apu-benchmark,3120-6.html
If you just play SC2, you probably want to get the i3. I personally like the i3-2120 because its clocked 200 mhz higher and only $3 more. Other games such as battlefield 3 multiplayer, diablo 3, deus ex, witcher 2, MW3 and others use more than 2 cores. So if you plan to play games that use more than 2 cores, you might want to sacrifice a little performance in SC2 for gains in other games.