Speed:
    * experiment with compiler flags
        Special thanks to Oscar Lesta. He suggested some compiler flags 
        for gcc that make a big difference. They shave 10-15% off
        execution time on some systems.  Try some combination of:
                -march=pentiumpro
                -ffast-math
                -fomit-frame-pointer

Reducing code size:
    * remove some of the butterflies. There are currently butterflies optimized for radices
        2,3,4,5.  It is worth mentioning that you can still use FFT sizes that contain 
        these factors, they just won't be quite as fast.  You can decide for yourself 
        whether to keep radix 2 or 4.  If you do some work in this area, let me 
        know what you find.