Depends on the core. If you're lucky, you could hit 2.8-3GHz. However, if you don't have voltage adjustments, your success could be considerably lower. There's some decent overclocking with some cheap motherboards, but others don't do as well. It just depends on the motherboard, CPU, and luck.
Whatever the case take it slow. For each overclock increase, fire up [google]Prime95[/google] for AT LEAST 12 hours, if not 24+, and let it run. If you don't get any errors, you're good to go. I'd go with small increases and not huge leaps. I'd probaly not do more than 10MHz increments on the Front Side Bus (FSB, and the only way you'll be overclocking).