You can do that, but your system will produce serious heat and major errors that would balistically pop from almost any random situation.
Running DX9 should not cause extra heat or errors. I've run DX9 without issue in Crysis since day one and have probably close to 1000 hours in CryEngine2 all up (Sandbox, SP, MP). As for the issue you experienced, put it down to bad luck and the inevitable case of hardware dying
yeah i did have the -dx9 in there but i took it out cause it wasnt working
Did you have a space between .exe" and -DX9? i.e;
Crysis.exe" -DX9, not
Crysis.exe"-DX9 or
Crysis.exe" - DX9