Iam using xfx geforce nvidia gt220 1gb ddr2 to how much extent i can overclock it?

i used evga precesion to overclock it as i cannot control the fan speed i set it to core clock-500mhz,mem clock 400mhz and the temp is currently48c.this temp is the temp when it is idle ..and importantly i did not fix any extra heat sink or anything in addtion to what i received with the graphic card..i have only one 120mm fan inside the cpu and no other fans|||well it really depends on your case and your cooling of our case, but since it doenst seem like you have good cooling, i would say a 10% overclock is your limit