First you need to reset the BIOS to defaults and then use the PII X 2. You can't set the processor itself to being 4 core unlocked permanently. It's the Mobo which does it.
So basically you need to reset the BIOS to defaults, remove CMOS battery, short jumpers, hit a switch whatever and then try to use the processor. Just boot into the BIOS after that. Don't try to change anything there, update to the latest BIOS and that's it.... your FX6100 should be good to go in that board.
Also after you update the BIOS again don't change anything in it, just boot into it, save and exit. Then shut down the rig.
Put in the new Processor and see if it works fine and shows up in the BIOS as it ought to.
If not, again reset the BIOS to it's defaults physically and then go ahead with the try.