how to choose an amp?

if i have a 400 watt sub should i get a 400 watt amp? or should i go more like 500 or something? or does it not matter as long as its 400 or over?

really depends on the music you play

+1, if you want to just make it simple, just match it up best as possible with the rms wattage, not the wattage (usually) advertised. i.e., you may have a "400 watt subwoofer" that really only puts out 200 watts rms, or an "1000 watt amp" that is really only something like 250 watts rms by 2 channels.

what I mean is, bass music is mostly electronic tones with very little dynamic range (hint: these are the songs you wanna do you burp with in contests), real music has lots of peaks but not as much average energy. As Vinny just said, if you listen to mostly bass music - try and get an amp that is close to the RMS power of the subs combined.