Nissan 240SX Performance Modification/Ignition and Fuel Management/Ignition
Spark plugs are a simple modification to a vehicle. If your plugs are dirty or old, it can make a big difference in how the engine runs. Also, quality plugs can improve cold starting behaviour, and improve fuel economy. Some may claim a power advantage from the spark plugs, but any improvements there are likely to be negligable. For boosted applications, spark plug choice can be very important. Under boost, the spark plugs should be slightly "colder" than stock, meaning the plug itself doesn't retain as much heat. This can aid in preventing detonation (aka motor death).
There are also after market ignition systems. MSD makes a product called the Boost Timing Master which just retards the timing under boost conditions. They also offer ignition coil upgrades (Blaster, Blaster 2), though many have complained about the MSD coils not giving good results on 240SX applications. There are also complete stand-alone units, such as those made by Megasquirt.