It depends on what you plan to do.
Your options are basically the dual core i3-2100 or the quad core Phenom II X4 965. For pure gaming without overclocking (an i3-2100 cannot be OC'ed), then either one should be fine. The core i3-2100 will give you slightly to somewhat decent better performance at 1680x1050 or lower resolution, but at 1920x1080 or higher, the difference narrows down / disappears.
The Phenom II X4 965 can be overclocked so it's performance will be better in games than the i3-2100. Of course, it is recommended you buy a heatsink capable of handling the extra heat.
If you do things that can take advantage of more than 2 cores, like encoding video with the x.264 codec, then a Phenom II X4 965 will perform better than the i3-2100 even if the Phenom is not overclocked.