How much underdrive pullies help sorta depends on the circumstances and how much power your engine's accessories draw.
A car with a 130 amp alternator, power steering, AC, etc will see a much larger gain than a car with 40amp alt, and no AC and PS.
Personally I wouldn't bother with the underdrives. The hassels arn't worth the 5-10HP.
The pertronix however is a great investment. I don't think it'll give you much for power gains, but it's sure nice never to worry about points again. I also noticed easier starts and a smoother idle.