Buy a new cpu tbh. Even highly overclocked that processor will still be slow. @ 1ghz it might get close to an 800mhz P3. If you must overclock it, just up the front side bus and increase the voltage to the cpu very slightly and see how far you can take it. If its unlocked, try upping the multiplyer too.
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...
Say the front side bus of the cpu is 100mhz, go into bios or look for jumpers on the motherboard and move it to 103mhz, let it boot windows, run benchmarks to make sure its stable, (prime 95 is good). Do this until you get an error just before windows boots or it wont boot up at all.
Then change the voltage to the cpu. Turn it up by 0.125 volts if possible but to be safe, dont go up by more than 0.5 volts. But make sure your temperatures are good. Below 50 deg C is ok celeron, ideally as close to or below 40 as possible.
When the voltage has been changed, the cpu will respond and MAY boot into windows, nothings for sure when overclocking though. If your memory is cheap, it probably wont go more than 7mhz above default. My memory is rated at 133mhz, its max is 142mhz! Thats where the multiplier comes in...
In the bios you may see a setting for this, may be set to 7 (7 X 100 = 700mhz). Changing it to 7.5 will get you 750mhz etc. Always make sure you know how to clear the cmos (bios) before messing with these settings. Make sure you have your motherboard manual and a spare cpu ready too hehehehe....
Just had a look at the manual, and I think you're out of luck as far as overclocking goes - your jumpers only allow 66, 100, and 133 FSB. While you could try setting your FSB to 100, I would be <b><i>extremely</i></b> suprised if your 700Mhz Celery could cope with 1050Mhz - even if you have liquid nitrogen cooling.
However, you <i>might</i> have some more options in the BIOS - probably under 'Advanced Chipset Features'. I can't tell for sure because the Manual helpfully just says something like
Don't change the settings in here. You can change the AGP aperture