1 - When it crashes. Most people use some burn-in software (such as the burn-in wizard in Sisoft Sandra) and leave it running for a few hours before they think it's stable.
2 - All modern PC CPUS get their end speed by multiplying the speed of the Front Side Bus (FSB) [which is usually also the speed your memory runs at] by an internal multiplier, e.g. your Celery is running on a 66Mhz Bus, so it's multiplier must be 10.5 - 10.5x66 = 700Mhz.
On some systems (but not intel systems) the multiplier can be adjusted, which of course results in a faster frequency.... But yours will almost certainly be locked, which only leaves the option of the FSB.
In this respect, you're probably more limited by what Bus options your motherboard has, and your RAM, than anything else. if your board has an option to have a 75Mhz Bus, then that would instantly give you 10.5*75 = 787.5Mhz.
If overclocking in this way results in instability, and your board has the option to do so, you could try <i>slightly</i> increasing the CPU VCore... But as the other guy said, just pick up a 1Ghz PIII or something, and it'll be LOADS faster than you'll ever get a celery to be...
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