The Type 4 P90 complex (as used on the Server 500 machines) is currently the hot ticket in PS/2 performance upgrades. Several different types of modifications have been reported on the newsgroup as being successful, including the one detailed here. This specific modification was done by myself on one of my complexes. Note that although I've got a stable, solid performing machine at this point I did experience various *quirks* in the past. I believe that the *quirks* have been resolved by adding cooling to the transistor on the Evergreen CPU adapter.
Modifying the P90 complex to run a P233 MMX requires two major steps, the first of which isn't really for the faint of heart since it requires that the 60 MHz oscillator on the precious P90 complex be desoldered and replaced with a 66 MHz unit. I must admit that I shed some sweat the first time I did this, but rest assured that it's not *all* that difficult to do if you have the appropriate tools and a steady hand.
A suitable replacement oscillator part is the Epson SG-615PH at 66.6667 MHz, which can be bought from vendors such as Digikey. Also, if you have an old Model 77 planar to waste you can donate its heart (the 66 MHz IBM-branded oscillator) to your complex as well.
Here's how a stock P90 card looks without the heatsink:
Between the processor and the cache controller is the oscillator, which gets replaced by a faster 66.667 MHz unit:
Note: I've used the 66 MHz, 66.667 MHz, and 66.6667 MHz oscillators successfully.
Once the oscillator is replaced, the complex operates at 66 MHz, much like the Type 4 P66 complex.
The second thing necessary to accomodate a P233 MMX CPU is (in my case) this Evergreen CPU adapter. It takes care of both the voltage-conversion and clock-multiplying duties.
Here's how the top and bottom of the Evergreen adapter looks:
Here's how it looks with the Pentium 233 MMX CPU on it:
The jumper settings have been documented HERE.
Here's a full view of the completed P233 MMX Type 4 complex (without heatsinks mounted):
Here's a close look at the fully mounted Evergreen unit:
(note that I attached the heatsink from the transistor on the Evergreen adapter to the CPU's heatsink)
Well, this is probably about as fast as you can go in a single-processor Micro Channel machine (for now). Your mileage may vary, of course. I'm still experimenting with some other stuff, but so far an attempt to run the AMD-K6 hasn't panned out.