Pretty simple to handle, I think
Just set your system to boot to runlevel3, not 5. Save your old XF86Config (or XF86Config-4, whichever). To choose which configurations to boot, you can tell XFree which config file to use with the -xf86config <file> option. Haven't used it myself, but it's in the XFree86 man pages. I don't know the rest, but there's a start for you. Someone's thought of it already!
Too tired....no sig.