The i3 is an absolute beast for a dual core. However, my 965 overclocked to 4,336 Mhz was actually up to 20% faster than my i5 2500k at stock clock speeds.
That said, looking towards the future, the more real cores the better it seems, as Crysis 3 has shown.
For $10 cheaper on newegg than the i3, the FX6300 is the processor to buy.
Here is the difference between a 965 @ 4,336 Mhz and an FX6300 @ 5,008 Mhz.
3DMark Vantage
965 @4.3Ghz - Score 21,851 -
http://www.3dmark.com/3dmv/4598862
FX6300 @ 5 Ghz - Score 25,082 -
http://www.3dmark.com/3dmv/4649609
The jump is just from swapping out the CPU.