Another issue:

While what you have probably will work as it is after you have implemented the above suggestions, you will start experiencing problems as your main loop grows. The proper way to handle serial communications (unless you are using 300 baud or less) is to implement a circular buffer in the ISR. Then in the main you drain it if it has contents. I have seen many cases of programs that 'suddenly' became erratic becuse of code like yours and the statement ALWAYS is "I did not do anything to the serial processing, so how can that be the cause"

Do it right from the beginning and your surprises will be pleasant.

Erik