There is no standard answer as overclocking results are all over the landscape. I would start with an FSB of 266, if that works go to a FSB of 275, if that works continue increasing the FSB by another 5 until you can no longer boot into Windows, then back it down 5, boot into Windows and run some programs that will stress the CPU.
If your system is stable I suggest that you back the FSB down another 5 and that should be a safe FSB for 24/7
Note that there are more in-depth ways of performing this task, however, this is a quick method.
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overclocking help