Do most shops in Venice, Florence, Siena and San G accept credit cards? My experience abroad two years ago was that this was the best exchange rate.
I realize the markets will need cash (Euros. How readily accessible are the ATM machines?

I have seldom had a problem using a credit card even in small shops. I haven't tried in a newsstand or tobacco shop, but other than that, credit cards are widely accepted in places that depend on tourism, and your towns do.

Florence, Siena, and and Venice have ATMs on the main streets

If I may jump on your post for a second to ask a related question... would it beneficial to get an ATM only card and not a debit card to use for cash purposes or should I get a pin # for my AMEX? I recently heard a commercial for a bank that does not charge any ATM fees, no matter which ATM you use. They also refund you up to 5 times for fees charged by the ATM itself. Would it be a good idea to open an account for our trip? I'm thinking of pick pocketers here and them getting a hold of our debit card and draining our accounts.

TIA,
Allison

Actually many of the market vendors in Florence do take credit cards.
They are connected to a power source in the building behind the stalls!

You should have absolutely no problem using credit cards in those locations, and similarly, no problem finding ATMs.