Here's my TP600X CPU upgrade story in detail, if someone sohould find it helpful...
My 600X was originally a non-speed-step 500MHz:
BIOS verison 11/30/99 - ITET55WW
Machine-type model: 26458EG
Slave controller version 10/08/99 - IHHT10WW
I'm Running win XP-sp2. Note! I have removed the Intels own speedstep program and use only Speedstep XP v.1.4.
Installed a 800 MHz MMC-2 speed step module to the old Thinkpad 600X (former 500MHz with no speed step)
"...running at 650/ 800MHz and the speedstep seems to be working, but with some hiccups. When XP starsts to load, it hangs, several times. By unpluggung and replugging the power supply cord the boot continues, but hangs again. Alternatively, I can press Fn+F4 and or Fn+F11 to continue instead of the power cord thing. When the XP is finally up and running, the Speed Step utility allows it to change the frequency, but it also locks up immediately. Pressing Fn+F11 unlocks the system and the CPU speed is then changed. But it freezes each time I toggle the speed. "
2nd upgrade: Changed the processor driver to:
-p3.sys -> boots ok both with (->800Mhz) or without PSU (->600MHz), but freezes if I unplug the PSU cabe when it has started with the PSU connected. Pressing FN F11 releases it.
No problems! Boots up and hanldes the un-pluggin and replugging of the PSU. Initial speed accordingly 800/600MHz. One thing though - if I boot without PSU the initial speed of 600MHz is the highest I get even if replugging the PSU.
Upgraded to a 850MHz MMC-module (which I bought earlier but didn't install when still having the unsolved boot problem).
Now my initial speed with PSU is 847MHz an it stays there even if I unplug the PSU. I can change to 650Mhz manually. When the battry starts to run low (below 50%), the CPU speed decreases rapidly 400MHz, 300MHz 200MHz ..150Mhz not sure how low it goes because still a new setup...
Very happy so far, now the Thinkpad is usable without hangups. With 2x250 MB RAM modules, a fast 80GB hardisk and a DVD-CD-RW combo it's quite satisfying. Anyway, I like the mechanics of the TP600X and it's just enough powerful to do everything you need on the road. The lack of USB 2 is a downer, but I have a PCMCIA Laptop Card USB 2.0 card in the bag if needed.
All in all, perfect...