for the Bios, I think thats just the way it is, you have to run a VGA bios when you want VGA and a normal when you want normal.
as for the dash, just replace the evoxdash.xbe (or perhaps default.xbe) with the default.xbe of your new dash. I use XBMC and i put the default.xbe of XBMC on C:\ as evoxdash.xbe. For XBMC you will also need to edit and put xboxmediacenter.xml on C too. The xboxmediacenter.xml file is very clear about how to edit it.