Fire proofing walls with Aluminum Foil Tape
The type-X drywall is a great idea as it is a great barrier to reduce heat transmission to combustible materials beneath, thereby increasing the amount of time it would take for the underlying materials to ignite when exposed to fire conditions. Use drywall tape and mud to finish the joints and call it good. As @Tester101 suggests, aluminum foil will do very little at structure fire temperatures, in fact I would bet the tape adhesive would catch fire and the tape would melt.