the 50x4 thing generally means how much power the h/u can put out. It's usually around 10-12rms give or take. So if you buy some new speakers and intend on running them off the headunit, it will sound pretty crap.

Generally, watts don't mean very much. Its the Rms output that is important. You always see budget amps promoting how much wattage they can put out ie 2000watts max. Thats not important. Generally speaker Rms output ranges between 45rms-100rms depending on brand, price, quality etc. So a reputable brand amp (2 or 4 channel depending on how many sets of speakers you are running, personally I'd go 4 channel for more flexibility) that puts out 80-100rms per channel would be sufficient. It's always better to overpower your speakers then underpower to minimisse clipping and frying of voice coils.

Hope that helps