Dear APC,
Well, that depends on whether or not you want to save/stay in your marriage. Because having an affair will probably not help your marriage. And if you already feel strongly about this man - you probably will have an affair with him.
Do you want to stay in your marriage? Is there something left to salvage? If not, move on, but do it the right way - by telling your husband it's over. Then you can reassess your feelings for this new man, without the shadow of an unhappy marriage hanging over both of you.
Whatever you decide, good luck.