What for? Your stock alternator will handle just about anything you want to throw at it. We're talking multi-thousands of watts from your amps. Swap out all your battery cables and alternator cables with 1/0 or 4 guage. That will help you electrical system alot.
If your car is just getting on in years and your replacing the alternator as a precausionary measure, then upgrading won't hurt, but again, it's hardly necessary