I think the answer to your question has to come from you because you're the one that knows the probs in your M and your W. Like Choc, I confronted my H and it didn't send him into the arms of OW. We separated for awhile, but it wasn't so he could see her. I confronted because I wouldn't be with someone continuing an affair, but most importantly because I knew my H was not ready to let go of our M and our family that easily. He would've been a cake eater, but not ready to move on with someone else.
Is your W someone that runs from probs? If so, confronting her might make her run from the discovery of an A. If you think though that she's more likely to worry about losing you (she did say you're a good man), then maybe you could ask her kindly about the card you found and what's going on with her. Come from a point of wanting to understand what she's going through rather than confrontation. You might just tell her that you know she's been in contact with OM and are willing to talk about it if/when she's ready. Then DB your butt off.
