Yes, if you are going to be running high output stereos you'll want a bigger alternator, but in reality not as much as having two batteries.
THAT IS A MUST. If you run any kind of stereo you need two batteries on a perko switch so you can switch from one battery to another in case you run one down.
If you do want a new alternator, I'm getting my new one from www.dbelectrical.com, they have marine grade alternators. And they have some big power pushers. I'm getting the 160amp since I've upped my stereo this year.