[mikegett]

The v6 has a 105 amp alt and the 4cyl. has the 102 amp alt. If you have the original altenator, deffinately think about a upgrade. If you have replaced it within the last two years, figure it into your budget but wait until you get the system installed. You may be able to get away with running it for awhile. It will deepend on how loud you use the system. If you run it at half volume you will only be seeing a quarter of the amps pulled 100 percent of the time.
The HU voltage and amplifier input should be kept close. You don't want to send 6 volts into a amp rated for only 2 volts input. If the amp rates for 4 and the HU is 6 you will probably be ok. Just keep the gain down as low as possible or you can burn up the inputs at high volumes. (INFO: the HU voltage increases as the volume is increased. This is why you set gains with the volume at near max)