You're much better off having a good battery, and heavy duty alternator. If you're lights are diming, it's usually a sign of either the battery is geting to the end of its life, or the alternator isn't doing a good enough job. If the battery is good, and the alternator is bigger than stock, a secondary battery would be a better option.
quoted from another site
If your lights dim, what makes adding another load device a good idea?
Current : 2 15s 149.5@56hz 2k rms Previous Trunk box 1 18 ,146.4@43hz 3500rms,Sony 12s 145.6@63hz