X86 os instability with 4gb ram

I have an Asus M2N68 with Athlon x2 Brisbane 4400+ and i cannot get any x86 os to work when i install my 2x2gb Geil kit. Windows xp will randomly freeze and vista won't even load. If i remove one memory then the pc runs fine. I searched a few solutions but nothing seemed to work. KB929777 did nothing, i underclocked the ram step by step till i got to 400MHz, i do have memory remapping enabled, I tested both memory modules with memtest - they're both good. Can you please give me any other ideas i could try? I know its possible to run x86 os with 4gb ram (though it won't use all of it) and
I just feel that I'm missing something...

Does it matter which of the memory sticks you remove? Or are you always removing the same RAM stick?

Thank you for the quick reply.

I have already tried altering the memory clocks. The memory modules are geil legacy 5 5 5 15 at 800 mhz. They are not supposed nor did I try to run them at 1066, i know that causes issues. What I meant is that I took them down to 667 - 533 - 400 MHz and also I tried with 5 5 5 18 and 6 6 6 18. Nothing worked. I didn't try to flash the bios yet (my fdd broke, buying a new one today), it will be the next thing I'll try. However, the later bios versions available on asus.com have comments saying they support new cpu's, nothing about fixing memory issues.

I updated the bios but it still doesn't work. It just turned the freeze that I was getting when vista was trying to start into a bsod 0x0000000a irql not less or equal.