Alternators should never produce more current than is needed by the electrical system. Provided that the alternator is working correctly, it shouldn't be a problem at any engine speed that doesn't cause other engine problems.
For sound systems, people get high output alternators, which can be designed one of two ways. It can have a greater maximum output, or else it can generate more output at low speed. Sometimes both. The amount of power that the alternator can generate comes from the crankshaft via the belt. This isn't "free" power, it comes from the gas that you burn. So keep that in mind. Your stereo takes its power from the crankshaft, so the more that the alternator is taking to keep the battery charged, the less that you have to drive the wheels. (Though you probably will never actually notice.)
Greater maximum output is fine if you're generally driving while you have your stereo cranked. Greater output at low RPM is better if you crank the stereo while you're sitting in the parking lot with the engine idling. (Like, say an IASCA show car.)
A higher output alternator is probably not required. The money is probably better spent on something else. A class "D" amplifier for your subwoofers will put far less drain on the electrical system, since it's so much more efficient. Upgrading the system grounds and power will do a lot as well. Most cars have a more than adequate charging system, so look at upgrading the alternator if you are finding that you are having trouble keeping a battery charged after you've installed your system. It probably isn't needed at all, but it would really suck if you upgraded for maximum output, and found that what you needed was more output at lower RPM, or vice versa.
Put the stereo system in first. Upgrade the power and ground wiring. Then see if you have problems. I'll bet that you won't.