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Ignition System
In terms of the ignition systems in these cars, there are two types: DIS, and traditional. The DIS is for the 3.3/3.8L V6 engines, and is a computer controled ignition system. Basically the PCM uses a cam and crank sensor to determine the position of the engine. This information is used to control the fuel injections and spark plugs. The traditional system (or what I call the traditional system) consists of the typical distributor cap and rotar, wires, plugs, and coil.. whereas the DIS lacks the distributor (hense the name DIS- Distributorless Ignition System...) and coil (the 3.3/3.5/3.8 uses a coil pack which is nothing like the traditional coils used in an ignition system).
In terms of improvement for these systems, your biggest gains per buck are with thicker spark plug wires. Since these cars use something like 7.5mm wires, 10mm or larger would produce better spark by having less loss through resistance. Performance or top grade spark plugs are nice to have as well. Especially in the case of the 3.3/3.8 whereas the engine is transverse and so close to the firewall that replacing the rear spark plugs is quite hard. In this case, it is nice to have platnum plugs which are good for 100,000 miles (+/-).
Timing is also worth considering. The 3.3/3.8 does not allow for timing changes (well, beyond what the PCM does). The timing on the 3.3/3.8 was adjusted automatically by the TCM via a knock sensor. Sadly this can not be adjusted by the user (well, without building something to hit the sensor with a hammer when you want a timing retard :P
It might be possible to use an after market timing controller on the 3.3/3.8- BUT, you'd need 3 of them (one for ea timing signal) and that can get expensive fast (~$100 ea- x3!)
Retarding the timing helps combat engine knock and is a helpful thing for when adding boost, going up hills, or towing. There are computers out there which will go between the coil (or in the 3.3's case, the coil pack) and the PCM and allow the driver to fine tune the timing. Such mods are costly (figure $200-500+) but can be worth while for some. Timing on the 3.3 can be manually controlled via 3 timing controller units or a later 3.3 PCM with the knock sensor curcuit and then by putting false knock sensor outputs into the PCM.
In the case of someone who wants to go all out, MSD aftermarket ignition systems are great as they allow for easier control of the ignition systems. MSD makes both a DIS and traditional system kits. Since the ignition is computer controled on the 3.3, replacing it also requires replacing the entire computer system! And that gets quite messy (however if you have the ability to replace the whole computer system, you lose the speed and rev limitor!).