Here's how I flash my GTX 260s.
First I always use my original BIOS as the base for the new BIOS. I save a back up then mod and save the BIOS as something else. I use short names like E20.
What you need is NiBiTor, NVflash 5.67 or better, and GPU-Z is optional. I save my BIOS with GPU-Z because for some reason NiBiTor doesn't want to extract it on Vista x64 SP1.
I create a DR-Dos boot DVD with Nero. I place that in my primary DVD drive. I keep Nvflash, GPU-Z, NiBiTor and all my BIOS files in a folder I created on my C
drive called NV
I back up my original BIOS, then open it and mod it. Save it as something else in the NV folder. I make sure the file extensions are .rom and not .bin. Any .bin gets changed to .rom.
Once I have my BIOS ready in the NV folder I restart with the DR Dos DVD in my primary DVD drive. I go through its prompts until I can navigate to my C
drive. Then I type cd nv\
. After that nvflash -4 -5 -6 <insert your BIOS>.rom
here. Hit Y
when prompted. Restart and you're done.
As always flash at your own risk. If you decide to do it, it's on you.
I flashed my cards because I wanted the core, memory and shader clock to be identical for SLi. Yes, I know that isn't necessary.
This isn't the only way to do it and it worked well and painless for me.