How many watts is your stereo?
Using basic electrical formulas: P=I*V where P=watts, I=amps, and V= Volts.
You'll probably need about 25 amps headroom to run everything else in your car (lights, ignition, wipers, etc), so if you had lets say a 78 amp alternator, you'd have 53 left for your stereo...so around 750w of a stereo would be able to be run off of that...though its always good to have some headroom. You'll have to do some wiring upgrades to handle this alternator upgrade. The stock charging wires are too small to handle more than 50 amps or so. There are plenty of topics on this.
I like the delco alternators better than the denso...I think they are better built, cheaper to replace, and are also internally regulated. I just posted a topic on installing an older style delco on a small block. check it out in this forum.