If it breaks you are screwed. The pistons will beat the valves apart and probably ruin the heads along with the piston heads, tearing up the block with broken parts is possible also. You could get lucky and have the engine die just after it bends a couple valves and not have too much damage.
Timing chains are a ***** to replace. The whole front of the engine has to come off. If you bent valves and whatnot, you have to take the heads off to replace them. If you have the SOHC, you have a chain on the back side of the engine too.