I'm planning to put a ribbon cable that's 6 inches long to connect two
boards.  One end has the 16F876 output ports and the other end will simply
have LEDs on it.

What should I look out for in this type of situation?

Where should I place series LED resistors, if it matters.  On the pic side
of the circuit or can they be over on the other LED board?
Should I be worrying about transmission line effects and what can I do about
it?
Should I be buffering/driving the LEDs instead of driving directly from
ports at that little distance?