Google is your friend. What the Gigabyte site say?
Let's start over. A motherboard supporting a 64 bit CPU needs a 64 bit BIOS. So any BIOS update for recent AMD or Intel CPU's will be 64 bits. Now, I was pretty certain of that, but I wasn't absolutely certain. So I checked.
" * CPU: An Intel desktop processor with support for Intel 64 is required. Consult the Processor Spec Finder to see which processors support Intel 64.
* Chipset: A motherboard with the one of the following chipsets is currently required to support Intel 64: Intel® 910, Intel® 915x/925X/XE, Intel® 945x946x/955X/975X, Intel® 965x/963x, Intel® 3 Series, Intel® 4 Series, or Intel® 5 Series Express Chipsets.
* BIOS: A BIOS must have support for Intel 64. Processor will not operate (including 32-bit operation) without an Intel 64-enabled BIOS." (my underline)
So, why do you think you need to update your BIOS?
Let me see, just be certain I understand, you are looking for a 64 bit SLI Gigabyte Dos BIOS update? As you have that worded, there is no such thing.
Please elaborate a little, as I cannot for the life of me figure out what you are asking.