Should I take into consideration the power that the headunit gives me, plus the power from the amp?

i want to buy some speakers for my 2008 scion tc (6.5 and 5.25) if i have pioneer headunit it will give me 22rms x 4. i want to know that if i hook up an amp to the speakers plus the headunit do the powers combine? (example: amp 50rms x 4 plus 22rms x 4 = 72rms x 4) is that the case??? if i know this then i will be able to determine which speakers to buy.