Please post full system specs.
However, from what you have said, you have an old 133FSB P4. That being the case, the '266' RAM option is 1:1, '333' is 4:5 (FSB:RAM).
So, if you are having problems passed 160FSB with RAM set to 266, your problem is not your RAM, since even at that OC, your RAM is still running below stock speeds. Now, if you have the RAM set to '333', that means at 160FSB your RAM is at 200 (stock speed), and anything above 160FSB would begin overclocking the RAM.
So.....if your RAM is actually at '266' per the BIOS and your getting errors past 160FSB, the problem is almost certainly either the CPU, or.... the chipset. In order to try going higher try increasing vcore and/or chipset voltage and see if you can go higher. Now I don't know off the top of my head what the high end of safe voltage are, but that is what google is for, and I would advise finding out. But really on a P4, as long as temps are OK, your not probably going to over volt it.
I would really advise doing a lot more research and reading before going too far. For the mean time, I would leave FSB at 160, set RAM to '333' and enjoy. Then, after you are more confident in overclocking try going further.... My $0.02.
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