The big three is upgrading the stock battery and alt wiring. Judging by most stock connectors and wiring Ive seen used, its not a bad idea to do any time you start adding significant draw on the charging system. //content.invisioncic.com/y282845/emoticons/wink.gif.608e3ea05f1a9f98611af0861652f8fb.gif
But as far as when exactly to upgrade it, or when to upgrade your alt, that's the $64,000 question. There is no exact answer. There are too many variables: your car's charging system, the charging system's current state, the stereo equipment you use and even your listening habits can affect if/when you need an upgrade. Lets face it, someone with a 1000 watt system who listens to it full blast all the time could need to upgrade sooner than someone with a 2000 watt system that rarely if ever bumps it at full potential. Best advice is to get the system going, probably go ahead and upgrade the big 3 if you are concerned as its not an expensive upgrade, and see what happens. Watch your voltage and see if your charging system keeps up, and not just by cranking the stereo and watching for headlight dimming, but over a period of time.