I would say that you don't need a new alternator unless or untill you are experiencing charging problems. Why replace a perfectly good alt if you don't have to. If you do have problems, you probably will not experience alternator failure without noticable signs beforehand. Additionally, if you are not playing your system for long periods of time or at all times when you drive, your stock alternator will probably hold up fine.
Additionally, if you are concerned, you may want to start by getting a deep cycle battery first.
However, if you insist on a new alternator, I would say to take the current from your stock alternator and add the currents from all the fuses from your audio equipment and any other power consuming add ons. This will give you more than enough juice to keep the battery charged.
One more thing. When I linked to your amp site, it seems to me that the power output stated was max output rather than rms. This may or may not be of concern to you.