Yup, don't just buy a set of tweets and tap it to the speakers. Get a whole new set of components. They are going to cost you about 140. I would go with alpine. That's what i'm using (alpine type R). They give good sound. Make sure you don't use directional housing. You want the sound to go from the passenger door to the driver, and the driver door to the passenger. For high pitch, you don't want direct sound. I have it installed witht he directional housing, it gives out to much highs.
As for an amp. Forget about it, your wasting money on something that doesn't need to be amped. A tweet doesn't use much power to produce sound since they are so small. If you get a amp, power up the the speakers instead, unless you get components where you power both the tweets and the speakers