I have lapped and matched sets of: 366's, 400's, 433's and 500's.
All hit the "wall", stability wise at about 581 to 583 or so. I have gotten the 366's to 605, 400's to 600 and 500's to about 650 but not stable.
The biggest "killer" for some strange reason seems to be Internet Explorer.
Example: If I try to access the Internet at or above 104 FSB using my 366's the machine either locks up or reboots. However, if I access the 'net at 100 FSB, no problems and THEN move to 572 (104 FSB) or 584 (106 FSB, using MBM5) then things are pretty stable. Attempting to load software above 100 FSB is not a good idea (I never do anyway).
So, to this point I have a VERY stable system using:
1. lapped and matched cpu's
2. active cooling on the BX chip set(mini-peltier and 486 HS and fan)
3. the largest cpu coolers that fit on the BP6, GlobalWIN FEP-32's
4. changed EC-10 from 100 uuf to 1500
5. changed out 2 voltage regulators
6. added a tandem PS (300 watt) just for the MOBO
I have been able to run multiple stress tests at the same time with the cpu's (both) at 100% and 594 meg of ram (out of 768) in use while cycling both hard drives, floppy and all 4 optical drives at the same time for 12+ hours straight.
Still, nothing REALLY stable (software wise) over 571 on a Ver .41 board.
My next step is to find a good Ver .40 board, completely re-cap it, mod it out and try again! (after all, my first Ver .40 went 1209 dual for 4 years, why not again?)
There is just something about this board that says I gotta try.
Besides, it gives me something to do, it's very cheap entertainment and it gets me away from the "honey do" lists my wife comes up with when I say "I'm working dear".

Regards,
jaybird