A guide for overclocking

[Evangelion]

hello i have this question i want to overclock my CPU

is an intel celeron D 420 1.6 mhz
my mother board is an asus conroe 1337-D667

i checked the temp and i see 40 with fanspeed, however i would like to OC for see results

but iam pretty nub about this thing of OC when i go to the bios setup it says something like this:

OverClock mode: Auto (options: auto; CPU,PCIE async; CPU,PCIE dsync)
GPU frequency: 200 (isnt activated havent touched anything, but the range is 200-500)
PCIE frequency: 100 (isnt activated havent toouched anything, but the range is 75-150)

however when i select the second option only the GPU F can be changed, and when i pick the third one dsync both can be changed

my question is wtf is GPU F and PCIE F? what to put async or dsync?

ty for any advice