Your both Celeronof 400 amd 500 are of Mendocino. Or called Celeron I. The max speed of the family is 533mhz. It used 250nm process.

Chances of going higher than 533 is very low. Even at 533, it was not stable also.

The heat is an issue. Maybe you can try with those today's latest heatsink to see if can cool down your Celeron fast enough so that you can go higher.

The Celeron II, the Coopermine family of 180nm process, were an excellent overclocker. The speed started from 533 until like 1000 or 1100.

You had a 600EB ? That is not Celeron, but P3. E means Coopermine/180nm and B means 133mhz FSB. Celeron has only 66 and 100 FSB.

For P3 of any EB speed, they were not popular among overclocker. Due to unable to clock higher. Not because of processor, but because of mobo limitation. And mobo limitation were mainly limited by the ram speed (pc100/133 sdaram).

Celeron Coopermine. When first released, were the hot cake among overclockers. 566mhz at 8.5 x 66, were easily overclocked to 8.5 x >95 to achieve >800mhz speed.

And better yet, eventhough Celeron has only 128k cache, compare to P3 of 256k cache, the performance at same mhz speed were less than 10% difference. But the price of a 566mhz Celeron was like 1/3 of a 800mhz P3.

Memtest86 is more on testing maximum hardware potential. Mainly the ram, cpu, memory controller and circuit boards.

While for SP2004, it is for testing maximum best optimum condition under Windows environment.

So, for me, I usually use memtest86 to find out the max that I can go. From there, I roughly know what is the max I should go in Windows. I usually will start at about 10% below the max Memtest86 speed.

This is due to if we oc too much and then run Windows, sometimes Windows or hard disk will be corrupted.

My D830 A0 stepping, oc to about 3500mhz also reaches >90c when full load. I use Termaltake Copper Tower with 100cfm fan. The air coming out, is so hot that sometimes, I want to be fast, put my cloth there to dry faster.

But my other D805 B0 stepping oc to 3500mhz also, using Ninja with 100cfm fan, only 37c at full load. The air is as hot as my forehead.

It looks like for Pentium D dual core, those with B0 stepping is not so hot.