Well, two batteries for a pc is not really nessesary if that's all you're running. A second alt is way out there!
Now, if you're running a couple thousanwat amps, multiple monitors, dvd players, xbox, etc, etc, all at once- a second battery is needed.
But for just a couple mid grade amps and a PC, one is fine.
There are other reasons though, for example:
Second battery to power just the pc allows it to survive cranking (or a tank circuit can be used),
longer run times with the engine off,
and similar uses.....
My stock electrical system runs my P4 and two mid grade amps just fine, no issues (I've done the "Big Three" though)