I have overclocked 3 of my last 4 Core2 processors to the outer edge of stability as defined by 24 hour Prime95 test runs. My systems are in good cooling cases (Antec 900's) with good heat sinks (one TRUE and two Zig Dark Knights. All systems have reached core speed limits before reaching voltage or thermal limits. And I leave EIST enabled.
GA-EP45-UD3P | Q9550 OC'd to 3.6 GHz (425 MHz X 8.5) C3 stepping
GA-EP45-UD3L | Q6600 OC'd to 3.6 GHz (400 MHz X 9)
GA-EP35-DS3P | E7500 OC'd to 4.1 GHz (373 MHz X 11)
GA-G41M-ES2L | E6500 OC'd to 3.87 GHz (352 MHz X 11) Motherboard limited