if you changed your visa type (from F or B to H4), and you have only a approve status, you need apply a new visa to re-entery US. if you did not change your visa type (mean when you came in US by H4 visa), and have a extened i-94 (on form i-797), you do not need a new visa. you can come back with in 30 days, but can not leave North America.
2. When I applied for permanent resident in Canada, I selected Toronto as my destination city. Can I land on the other city?
you can landing any where in CA.
3. How much money should I take? Is a U.S. bank account statement enough? Do I have to open a Canada bank account and deposit the amount that I stated in my applications in it?
Bank statement is enough, no need to open an account in CA.
4. I have seen some people on this website gave their Canada friend's address to the Canadian Immigration Officer for mailing the maple card. But I am curious why they bother to re-enter Canada to get the Maple card. Why didn't they ask their friend to mail the card to them at US?
they warried re-entry CA for get Maple Card is because after Jan 1, 2004, you can not leave CA address for mail Maple card, they have to pick up the Maple card by person with two IDs. (I am not sure about this change, I landinged before Jan 1, 2004, and leave a CA address for mail Maple card).