Ideally, it would be better to get two amps (one 4-channel, and one mono sub amp). Then you can use the 4-channel amp to power all 4 speakers, and use the mono amp to power your subs. But then, you MIGHT need to install another component: a capacitor (which'll cost more money, of course).
However, if you're on a budget, or not really an extreme audio-head, then you should be fine with a single 4-channel amp, which is what I have right now. Here, you can use channels 1&2 to power the front speakers, and bridge channels 3&4 to power your sub(s). I think this is loud enough for me, at least for now (until I have more $$$, of course). *lol*