Hi again
Looking at the manual, the settings in the bios should be 133 for processor speed and 133 for Ram speed.
The difference between the XP1800 and XP2400 is solely down to the multiplier settings (the 1800 runs at 11.5x while the 2400 runs at 15x133).
In other words either:
1. Your bios is not detecting the chip correctly and you will need a new bios upgrade to allow for the multiplier/chip recognition.
or
2. Your processor has blown one of the multiplier bridges that control its speed setting in it's recent 'lots of volts up the wrong place' scenario.
The manual says that it supports upto XP2600 so I'd guess at option 2 for which there is no easy fix.
What does the multiplier say in your cmos?