calculate stereo rms wattage?

I was looking into an underdrive crank pulley, and they say to watch out if your stereo is over 400 watts rms. What's the best way to figure this out? maybe someone can give me an estimate.

head unit: pioneer deh-p730 (I guess that's ~22x4 rms?)
speakers: 4 sony xplode 3-way 6 1/2" (160w peak, don't know rms)
amp: phoenix gold xs4600 (600 watts, 4 channels)
sub: 12" Boston Acoustics Pro Series (specs say "up to 1200 watts")

*amp is bridged for sub, thats all it runs. I was thinking hooking the rear speakers up to it too, would that be a dramatic increase?

thanks all

The total wattage in a system, is the sum of all the system's power (RMS). Don't mind the peak power too much.

ie. If you have an 200W amp driven system, it's total power is about 200 W. (The head unit is not included because it does not output any power to any speaker... just a DC voltage to the amp.)

so since my speakers are being run by the head unit and the amp is 2 channels bridged @ 4ohms (runs 300wrmsx2 from phoenix gold)apparently, I have:
4x22=88
+300x1=300
--->388wrms

now, if I ran the two back speakers on one of the remaining 2 channels, would they be running at 4ohms also? PG website says 4 ohm stereo is 75x4. then, would I have(?)

2x22=44
+300x1=300
+75x1=75
--->419wrms

or should I run each of the rear speakers off of it's own channel (since I have 2 remaining), which would add another 75 watts to the above (which is bad for what my intent is). Thanks.