I'm not great at this kind of stuff, but I feel it's important for you to know the extent of your wife's affair. Or...maybe not...if you knew it was a PA, would that change your behavior? Would it make you want to make a decision (i.e., to leave, to separate, to divorce)? If it's not going to change the fact that you want your marriage to work, maybe it's not worth knowing.
The question for me is: How are you going to build a connection with your wife so she won't seek it elsewhere? I suggest checking out http://www.marriagefitness.com. Mort Fertel also has a book called Marriage Fitness that gives great tips on how to create a connection with your spouse.
Basically, he says that you have create an atmosphere of loving kindness. Table the negative stuff for now...put it away and focus on creating a connection. When you feel like there is goodwill and you feel kinder and gentler towards her, then work on your issues.
As I said, I am no expert, but I hope it's something!
ntl
Me: 30 H: 32 Dating 10/96 Married 8/01 H PA's: Summer 97, 12/06, 5/07-10/23/07 My Saga