as mentioned a crystal cannot be used at that frequency, only an external oscillator. this is confirmed by my own tests. the fastest i was able to get with a crystal was 22mhz-24mhz depending on the cut and caps.
this is great for fun but be aware there is potential for damage. not from heat like overclocking a pc cpu as many think but damage to the flash structures. atmel makes it clear flash and eeprom cell programming is not recommended over 10% of nominal clock speed. i verified that spm at highest limit does indeed result in failure. constant erase cycles caused measurable retention time deficit in minutes or hours overclocked while same routines within spec went for weeks. few tens of thousands vs at least couple million.
moral of the story: if you overclock dont write to flash.