ok depends what type of mod chip you looking at and what car you got??
resistor mod chip - avoid as everyone says it just fools the system and make the car run richer.
diesel tuning box - good idea if you have a diesel can increase bhp up to an extra 30 bhp and shed loads of torque. to gain full advantage of this go to a garage to get profesionally set up on a rolling road.
chip - needs to be done profesionally by a computer and rewrites the
program to give you a gain in power and torque usually about 5 to 10 bhp depends on engine size. i had a guy do my old beemer and gained about 12 bhp(est) due to adding other parts before rolling roaded it.
hope this helps