What BIOS you use has shit to do with what chip you use. You can use any BIOS on any chip (with the exception of the X3 BIOS, which can only be used on the X3 chip.)
The two factors in BIOS choice are preference, and more importantly, Xbox version.
If you have an older box (pre-April 2004) that is a v1.0 to v1.5, you can use almost any BIOS you like. If you have a newer box, v1.6 or 1.6b, you HAVE to use the Evox M8 BIOS (and then choose which version of that
you need, as the 1.6 and 1.6b have different RAM chips and thus require different BIOSes).
So, if you have a v1.0-1.5 box, I highly
recommend the x2.4981 BIOS. It is modifiable, stable and has excellent IGR. The newer x2.4983 is fine, too, but can't be modified.
If you have to use the M8 BIOS because you have a v1.6/v1.6b box, then you'll need to know that X2.6 chips in v1.6 Xboxes need to have the BIOS flashed onto Bank TWO rather than Bank One.
I'm not sure if the X2.6 CE
has this issue, but you should be aware of it.
Also, you'll need to resize your BIOS to 512kb to fit on the BIOS bank (the X2.6 has two 512kb banks...), which you can learn how do here
Note: The X2.x chips are very much all the same, so that guide, although it references older versions of chips and FlashBIOS, is still spot on when it comes to resizing your BIOS and flashing from disc.