Definately add an external amp if you can. It will just make your system cleaner. You will add better power and possibly adjust the frequency sent to the speakers if the amp you choose has an electronic crossover. This will give you better sound at higher volumes and less distortion from unwanted bass in your smaller speakers. You dont need anything super fancy.. Just a nice 4 channel amp to add. It will make a big difference!
http://www.crutchfield.com/S-TZwmI1FDjEO/cgi-bin/ProdView.asp?g=120&I=236TC4004
Here is a good example... 50 watts X4.. MTX makes a decent amp and it is under $300.00.
Alpine MRP-F250 is another nice piece for $200.00 40 w x 4 with built in crossovers.
Best of luck!