Honestly, I've talked to a few people from www.overclock.net
who have OCed that computer pretty high on stock cooling. They've taken it from 1.8 to 2.4 or so safely. Just apply Arctic Silver 5 rather than the conventional thermal pad, make sure you have good ventilation, basic fans, and that you watch your temps. Worst case, you drop it back down.
Aside from the Opteron, that CPU is supposedly the easiest to overclock.