no. there's a good chance it will make it worse. for half the price of a cap you can do the big 3, which will help more. a cap is only useful when you already have a good electrical system.
what car do you have? most companies you have to e-mail for a price. try http://www.mean-green.com. also what amp do you have? you might not need a new alt and maybe just the big 3.