A number of i5's out there.
As avenseth12 stated if it is an i5-2xxx (SB CPU) then he is correct on K verse Non-K. The non K can be Slightly OCed by raising the Bclk slightly (ie 100 ->105 MHz). HOWEVER this is Not worth it as you would not see any gain in performance and rasise the possibility of instability.
Also true for IV
However the base frequency you quoted is for an i5-750 (2.66 GHz) which can be OCed to 3.2 GHz farily easy.
Have to run, but if i5-750 see: http://www.google.com/#hl=en&sclient=psy-ab&q=i5-750+overclocking&oq=i5-750&gs_l=serp.1.1.0l4.4552.5223.0.8220.127.116.11.18.104.22.168.125.0j1.1.0.les%3B..0.0...1c.FLICq037WhY&pbx=1&bav=on.2,or.r_gc.r_pw.r_qf.&fp=2ab95d013a2a4bef&biw=1236&bih=624