Yes, first invest in a factory service manual and a digital multimeter! You can get a cd-rom version from www.ideal-z.com The ideal-z site will have a link to Courtsey Nissan, whom you will actually get the manual from. While at the Courtsey site, pick up a copy of the Ztherapy Understanding FI video. It too will help explain things. All the info you need is in the engine fuel section. In the first few pages you will see a drawing of the engine compartment which will help to id the various sensors. There is really nothing to adjust on the cars... besides the timing, idle speed and the adjustment screw on the air flow meter (which you should not touch, you can make matters worse and besides it will not help your problem). Look for corrosion in the wiring connectors to the water temp sensor and the bullet connectors located about 1' back from those connectors. Also test the afm (air flow meter) per the fsm (factory service manual). Look for any vacuum leaks like holes and cracks in the boot between the afm and throttle body, all other hoses, ect. Make sure all FI connectors are tight on the sensor, all vac lines are hooked up, cold start valve is not leaking ect.
If it's really running rich, hotter plugs is not the way to fix it. The hotter plug might not foul as quickly, but the mixture will still be too rich. The things John told you to check are right on, although some things, like vacuum leaks, usually make it run lean.