Using most current BIOS revs, you can do this rather simply. The newer X2 BIOS is the one I use.
All you need to do is rename a few files.
First off, I'm assuming your install of EvolutionX caused you to rename your evoxdash.xbe file to xboxdash.xbe and your original xboxdash.xbe to msxboxdash.xbe.
1) Flash your BIOS to something new. (X2 4976.02 might be a good choice.)
2) Rename xboxdash.xbe (the renamed evox executable) to evoxdash.xbe
3) Rename msxboxdash.xbe (the original xbox dashboard executable) to xboxdash.xbe
4) Make sure your HDD is locked.
Viola! Disable your mod and when you boot, it should come up to the original M$ XBox Dash. Enable the mod, and *POOF*, EvolutionX Dashboard.
Hope this helps!