if you have a 50w per ch HU and your not planning to use a extern 4ch amp to power your speaker you should avoid getting too powerfull speakers, like over 50 or 60 RMS cause depend of the HU they will have tendency to distortion and they will not be at their optimal quality.
Depend which kind of sound you like, if its bass that you want, dont spend so much on speaker, get something like pioneer 6X8 without extern amps and get a amp/subwoofer, it will more worth it.