Depends on the dual-core. Also depends on air flow and the stock cooler/custom cooler your using. In theory you can get .2mhz to .3mhz without any fear.
Also cheaper systems wont overclock as well as the medium-high systems since they use cheaper parts and what have you. A lot goes into mind when overclocking.
Some chips such as my Q6600 most can get 3.0ghz (stock is 2.4ghz) without any effort.
Also keep in mind, that if you're running a pre-built machine, companies like dell and HP have a tendency to lock the BIOS of their mobo's, rendering OC'ing impossible.