x2 4000+ overclock question

Hi all,

I am new to here and not a native English speaker, please forgive my bad English

I would like to ask with the following system, what is the estimated safe overclock rate:
--------------------------------------
Athlon64 x2 4000+
ASUS M2N-E motherboard
Kingston DDR2-667 1GB x 2
Western Digital 120 SATA-2 HD
Sapphire X1950Pro 256MB PCI-E
--------------------------------------

Moreover, since I don't want to overclock the memory, I tried to set DDR2-533 in the BIOS, HTT 4x, FSB 254, multiplier @ default 10.5 giving a speed of 2667 (memory @ 333.3MHz) but cannot POST (long beep sounds and black screen).

If I change the memory speed to DDR2-667 it can POST and stable in Windows or 3D games, but the memory will be running at 381MHz and thus being overclocked. Any idea/suggestions?

Thanks!!

Yes, I can just leave it if it is stable, but the point is I don't want to overclock the memory.

So I am worried if even I replace the DDR2-667 with DDR2-800 in order to keep the memory running under specs, the motherbroad will not allow me to use DDR2-667 to boot up and I must use DDR2-800 to boot up (thus overclocking the memory to 444.5MHz again)