vpapanik:
Thanks for the input Nick !I stick to the 8 MHz Arduino Pro, because it runs @3.3V which is pin-compatible with all my peripherals. However, it is safe to raise the clock to 12 MHz (according to datasheet specs), or even overclock it to 16 MHz, which I have read that it doesn't cause any problems under normal temperatures. I would surely consider it as an option.
12MHz is going to cause you headaches compiling a compatible bootloader - people do get away with 16MHz at 3.3V, but note that there is no guarantee that future batches of the chip will work reliably at that speed. Of course future revisions of the chip might officially support higher clock speeds anyway...