I went with the creative inspire 2.1 speakers and two Sony xplod speakers. I'm very pleased with the results.
Here is a video that should help:
Is it as simple as just cutting the wires and wiring them straight to the car audio speakers?
Yes, it basically is this simple. I had no trouble hooking up the car speakers to the creative system and I had never done anything like this before. All of the ones you are looking at will work. The creative system and the Xplod speakers are both 4ohms, but the Pioneer speakers I couldn't find an impedance. I'm pretty sure most car speakers are 4ohms so your probably alright.