Which bios do I have on my Z64?

When you power on the Z64, the bios version is shown at the display.
The number before the letter e.g. 2.20 means that the bios version 2.20 is flashed.

The newest official bios is 2.17.
The newest inofficial is 2.20, it allows the use of Compact Flash and Smart Digital cards.

Readme of the z64bios220.zip:

Based on unofficial 2.18d Z64 Bios.
Created by SilverFox, April 2007.
Changes from 2.18d:

Onspec's "internal.sys" ide driver has been replaced with moddified version "newide.sys" that will allow the use of Compact Flash cards via a CF-IDE adapter. CF-IDE use allows for 100% silent operation, with lower power consumption giving reliable use in a Z64 with no extra power supply (I had problems on 3 z64's running a 2.5" HD off its internal power).

Note1: Iomega Zip and multiple partitions etc.. left intact from 2.18, so z64start.bat, and automatic joining of partitions should all still work.
Note2: normal IDE HD's should still be 100% fully supported.
Note3: Compact Flash must be formatted just like IDE HD - i.e. from a Dos5 bootdisk or partitionmagic etc.. Formatting in windows won't work.
Tested on z64 HW3.