When did ABIT release their first motherboard that allowed for
accelerating the front-side bus in BIOS, and what was the board
called?

Hello, 

I did some checking in to Abit boards for you.  I wasn't totally sure
if you wanted the first board that allowed specific FSB control or if
you wanted the first board that allowed BIOS driven CPU setup, so I
have found both for you.

First, and probably the one you're looking for, is the Abit BF6, the
first to support specific control of the FSB speed setting using Soft
Menu III in the BIOS.  This is a Slot 1 board running the Intel 440BX
chipset, supporting PII/III and Celeron.  It gives you the ability to
set your CPU FSB clock from 84 to 200 MHz in 1 MHz increments.  The
board was released in late 99.  Manual is available here:
http://fae.abit.com.tw/eng/download/dlmanual.php?name=BF6&file=bf6.pdf

This first board to use Soft Menu, which allowed you to configure your
CPU settings without DIP switches or jumpers, is the AR5.  External
clock speed could be set to 50, 55, 60, 66, 75 MHz with a multiplier
of 1.5, 1.75, 2.0, 2.5, 3.0, or 3.5.  Whether overclocking would be
possible with this board would depend on the CPU you were using. 
Release date was mid 97 or so.  The manual for the board can be found
here: http://fae.abit.com.tw/eng/download/dlmanual.php?name=AR5&file=ar5e.exe
.

The first P2/Celeron board with Soft Menu is the Slot 1 VT6X4.  This
uses the VIA Apollo Pro 133A chipset and is quite flexible on CPU
settings.  It supports an external clock of 66 to 150 MHz at about
fifteen different settings, and allows a multiplier setting from 2.0
to 8.0 in increment of 0.5.  This board was released in early 2000 as
far as I can decipher, though I suspect that it may have been very
late in 99.  The manual for this board can be found here:
http://fae.abit.com.tw/eng/download/dlmanual.php?name=VT6X4&file=vt6x4.pdf

Hope this is what you were looking for.

Perfect. Just what I needed. Thanks.