The numbers indicate the CHDK source version they are based on ...
That means CHDK-sx20-100f-0.9.9-866.zip is the newest and should be the most functional.
If you can not get it to run, why not ask your friend (with the sx10) to prepare the card for you:
Just let him prepare your card the same way as his own (using your friend's sx10 files),
make the card bootable, remove the (sx10) PS.FI2, and then replace the (sx10) DISKBOOT.BIN on the card
with the (sx20) DISKBOOT.BIN from your CHDK-sx20-100f-0.9.9-866.zip. Then lock your card, put it
in your SX20 and start.
You will then have all the 'extra files' you asked about - they are the same for all camera's
hope that helps,
wim