Can it be overclocked? Yes, but only by +400MHz Turbo.
The standard maximum Turbo speed for i5-2500 is 3.7GHz. It can be overclocked to a maximum of 4.1GHz via multiplier. You can also overclock the BCLK by a limited amount. If you're very very very very lucky, you might get 4510MHz total (41 x 110MHz BCLK) -- most cannot get BCLK that high though.
The base clock of the sandy bridge CPUs is tied into everything and can mess things up. As far as I know it can only be increased by about 5-10mhz, unlike previous gen where people take it from 133 to over 200.