Well, in this case you're risking the Board to go up in flames (almost literally, as the Voltage Regulators might blow, or 'burn out' ).
If the Bios offers 1.5,1.6,1.7V, these are indeed only meant to give a slight boost to 1.5V AGP cards (OverClockers love those options).
Auto should use default 1.5V.
There is a
really remote chance, that Auto might indeed give you 3.3V (yet unlikely). The only way to confirm that would be the Hardware Monitoring of either the Bios, or Monitoring Software. But if those don't display the AGP Voltage, assume worst case, only 1.5V supported.
The Bios release should always be the latest one you can find, if you decide to flash. There's is no need to 'step-by-step' flash them, just do it once with the latest one.
It seems your Board comes in different Revisions and Releases.
You must flash the Version matching your current Bios Software (either AMI or AWARD). It should tell you at Power-On, and in the Bios itself.
The Flash Programs are usually smart enough to see if there's a mismatch, but I wouldn't rely on that.
Note also, that a Bios Update normally cannot give you 3.3V AGP if that wasn't in there in the first place. This is a Hardware limitation, and the AGP slot probably has gaps/bumps (on 1.5V and 3.3V Signaling Pins) that should physically prevent a 3.3V only AGP Card to be installed onto a 1.5V only Slot and vice versa.
Unfortunately, many of the old 3.3V Cards fit anyway into these 1.5V Slots, due to such matters being simply disregarded in the old days
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From the current look at things, it seems that you're really hosed
You either look into PCI cards (in case it MUST be a Voodoo), or choose other 1.5V AGP cards...
Your 25 Voodoo3's probably have to end up at Ebay in order to minimize the losses I guess
Since I don't know what purpose those Voodoo3's were meant to have, you'll have to see for yourself, what other PCI/AGP cards might suit your needs...