I think you may be mixing up wanting to change the BIOS with simply wanting a different, more functional main dash.
Replacing the EvoX dash is easily done, the main choice being between UnleashX and XBMC. The recommendation is to have one installed as the main dash and the other as an app. EvoX can be kept as an app too.
The SID2 you say you used is VERY old and what you need to do now is find HeXEn (Heimdall's Xbox Engineering disc), burn it to DVD-R at the slowest supported speed and use it to update your whole installation.
It will update everything with clean files to the current best, safest (Shadow C protected) softmod devised and install either dash as part of the process. It has many other useful tools.
I'd also find AID (Auto Installer Deluxe) v4.53, the last version released, and burn that to disc in the same way. It is a fantastic resource. With AID and HeXEn you probably won't need anything else.
BTW: AID/SID can actually be installed as an app from the disc and, although you can't do it from the disc version, there is now a HeXEn app adaptation you might be able to find too. They can both be very useful when installed like this if you have the space on the HDD.
Do all this before upgrading the HDD (see the tutorials here for how to do that). Make sure you have backup of the eeprom and anything important if not done already FTPed or otherwise copied to an external device ie. PC.
