exactly, it's not necessary to, but the improvement in sound quality makes it well worth it. the good thing about building a sound system is, you dont need to do everything at once, most things will work as direct upgrades. my dc4 ran upgraded speakers w/o amp, and it's decent, but in no way can it compare to my amp powered infinity's in my da. even upgraded HUs have their limits, be sure to stay within it, or you can end up damaging the upgraded speakers.
as for infinity's the sound it offers is crisp and clear. it's response to bass isnt very high, but if you're building on clarity, infinity's are a good start. if bass is what you're looking for, get a sub.