Acer Aspire E380, Need Help in OC

[Enmity]

i recently got an acer aspire e380-ed423A and i'm new to overclocking

i don't want to ruin my warranty yet... so can anyone with the same or similiar system tell me what's the optimal amount i can overclock the processor without opening it and installing a heatsink or another form of cooling?

 

[Enmity]

also i forgot to say its an amd athlon x2 4200+ (i wanted to get an FX but w/e.... )