I have a Fusion MS-UD750.
There is a "built in" amp inside the head unit... so it supports four speakers just like you already have. This allows you to have 2 "zones". It also has an output for a subwoofer.
Are you getting a package that comes with an additional amplifier? Dedicated amplifiers usually produce sound of higher quality than the amps built into the head unit but I find the sound of the built in very very good. Also, I don't have the hearing I did when I was 18.
So, no... you do not need an external amp unless you are:
a) going to drive a subwoofer
b) going to have more than two zones
c) want to blast your neighbors out of the water
or scare away whales
With that said...
We are buying
two compact speakers and a matching sub. The sub has an internal 4 channel amp. You hook the speakers to the amp in the subwoofer rather than to the head unit. This has two advantages:
1) You can set the frequency cutoff.... what frequencies go to your sub and which ones go to your satellite
speakers. I don't think the head unit has a adjustable frequency cut. I may be wrong but I have not seen it.
2) If you head unit is in a place that is convenient to access but does not have good ventilation you can help it last longer by avoid heat build up because its not being used to amplify.