An upgraded ignition will typically gaive you a small but noticable increase in mileage and power. The stock ignition system in a 300ZX is quite good, upgrades are not necessary, nor are they worth the money for most people. If you are doing a normal tune up though, do spend a few extra dollars on quality components. This means Nissan cap and rotor, NGK or other quality brand of wire(accel makes a very nice set of wires fitted for 300ZXs, and they're yellow too!), and quality plugs. Quality plugs are or NGK. Splitfire plugs are not quality. If you are replacing the coil, replace it with either a Nissan coil or a quality aftermarket one, not a cheapie auto zone piece of crap. Nissan coils are expensive though. I'd reccomend you get one at a junkyard should yours fail completely and you want to keep it stock. Periodic coil replacement is not necessary as coils generally don't wear out, they just fail if anything, but complete failure is rare.
If you are going to upgrade your ignition there are many possibilities. Popular choices include a Jacobs system, a MSD box, or a Crane HI6. The Crane HI6 and MSD are ignition boxes that you connect between the stock computer and coil, and a good 12 volt source. It charges a large capacitor between cyclinder firings and discharges the capacitor to the coil when the computer fires. This puts more energy into the coil, thus more energy comes out. This results in a longer and hotter spark at all engine speeds and allows you to significantly increase the spark gap, making a much hotter spark. The Jacobs system is similar I assume, although I have no experience with it. Expect to pay about $150 for the HI6 at your local speed shop, they are very popular with the Mustang crowd. I would reccomend getting a quality coil at the same time. This will allow the system to put out maximum power as well as replacing your tired decade old coil. A quality coil shouldn't be more than $50 or so. If you know what you're doing and don't mind the side effects, you can also use solid core plug wires, available at any decent race shop by the foot.