1. The range of human hearing is 20Hz - 20,000Hz. Subs generally cover the range of 0-150Hz. Less than 1% of the total range. Keep this in mind when coming up with a budget for your system.
2. Installation and set up are more important than the quality of your speakers. Plan to spend aprox. 50% of your budget on the install, if you want it to sound good, keep the music inside your car, and not rattle or resonate.
The parts you need are:
4 channel amp
Speaker mounting brackets
If you have a larger vehicle, rear speakers would be beneficial. Although many purists will say unnecessary.
If you are looking to cut corners or save money, do with cheaper subs, speakers, and the mono amp. Don't go cheap on the install, wire, damping material, brackets. etc.
You could also downgrade the 4 chan. amp to a 2 chan. and use crossovers, but that makes upgrades more difficult.
Also, it's been a while since I've been in the market but ebay has proven in the past to offer exellent savings. Also do the install yourself, it's fun and will save big money.
Why don't you tell us how much $$ you have to work with. What your goal for this system is and we will help you spend your money the right way.