|"i have a core 2 duo e4400 2.0 ghz"|
OK, that's one of the lower end C2D Allendales that only runs at 800MHz FSB.
"and the mobo its intel d946gzid"
Could it be D946GZIS?
Unfortunately you have an Intel board. The vast majority of Intel boards are NOT overclockable thru the BIOS - your board is no exception. You could try the software approach but I don't recommend it. If anything, get yourself a decent P45 based motherboard, dump the DDR2-667 in favor of DDR2-800, then you could easily overclock your CPU to 2.8 - 3.0GHz at 1600MHz FSB (the higher the FSB, the better the performance). But as it sits, you're stuck in the mud with your configuration.