The factory sound system, I find, just isn't very powerful. I had an Infinity that I cooked with a voltage excursion due to a solar charger (sad day) that I replaced with a Pioneer DEH-1300MP, and the change is night and day. Much more power, and sound was comparable to the quality of the Infinity.
That being said, you can get 4-channel amps that are designed to do exactly this. More than about 25-30 W RMS/ch is overkill, I think, if you really don't want a deafening system. My receiver puts out somewhere between 14-22 RMS/ch depending on who you believe, and that's more than enough to give a VERY accurate representation of Springsteen's masterful version of "Prove It All Night" at Passaic, 1978, at over 90db. That's a hearing-damage level. I have a suspicion that you may even be able to get away with fewer watts, since the new 5 1/4's are probably more efficient than the old stock speakers that I have. (RMS, by the way, is a "realistic" power rating. Just because I have a "200-watt" stereo (4x50 max) does not mean that's the kind of power it's pushing at any given time. 4x50 is the absolute highest peak that the chips can push without destroying themselves, and it will not be a pretty sound when those peaks hit. If you average out the power that the chips are putting out at max volume, it'll be roughly that RMS value.)
Crutchfield would be my go-to on this.