Thanks all for your input and guidance.

I called my 3 potential CC companies today. It was interesting.

1. My Credit Union where I have a Visa debit card (checking) and a Visa CC.
With both cards they tell me I'll pay .80 % on International US Currency transactions and 1.00% on non-US Currency transactions.
They define International US Currency transactions as those where the merchant in RIO bills me in U.S. dollars. All ATM cash advances and charges in local (Reals) would be charged the 1.00% Foreign Transaction Fee (FTF)plus whatever the ATM owner charges for the withdrawal. This Visa is CO-OP affiliated.
This is interesting because the wiki (previous post) says to avoid accepting charges already converted.

2. My MC Citi Advantage Platinum card says they'll charge me 3% whether I use it in an ATM or at a merchant. This MC is Cirrus affiliated.

3. My AMEX SPG appears to be the worst at ATMs at least. They tell me it'll be a 2% conversion fee in both cases. If I use an ATM for a cash advance it'll be an additional 3% (5% total!).

I also checked into acquiring the Reals before departure since that seems like a smart thing to do so that when you get there, tired from the overnight flight, you don't have to deal with the conversion right away. However, I've found the exchange rate offered by AMEX is only 1.26 per USD (where today the exchange rate is 1.66 per USD) and they charge about $15 per transaction plus something like $12.95 to ship it to you. Hardly a deal even worth considering in my opinion.
I have yet to locate an establishment in the Denver/Boulder, Colorado area where I can go in person to do this. I'm somewhat resigned that even if I did, it would be similar to AMEX, less the shipping charge.

So, I'm going to primarily use my Credit Union cards for this trip. But, I plan to use all 3 at least once at an ATM and once at a merchant. I'll post my experience when I get back.
From what I've heard, the trick is to check the exchange rate when you get the bill from the merchant and to look for either the CO-OP or CIRRUS logo on the ATM machines.

Again, thanks for the input! I'll post my results when I return from the trip.