I'm thinking of exchanging money at our bank before we leave for 3 days in Bangkok and 4 days in Phuket. Any suggestions on about how much money we might need? Or is it better to exchange money when we get there?

You will get a better exchange rate if you buy the Baht here in Thailand.

It will definitely be best to exchange here.

Traveler's checks are a safe way to carry "cash", and receive a good rate of exchange. Most vendors won't accept them, but exchange booths are everywhere. TCs are charged a 33 baht exchange fee.

If you have an ATM card, with a 4 digit PIN, you can use machines to acquire baht. There is a 150 baht fee for using Thai ATMs. The exception is AEON ATMs, which assess no fee. Unfortunately, AEON machines are a bit few and far between.