Dear yilmau: If an axillary lymph node dissection is planned, it should happen before any additional therapy. Otherwise, the information may be incorrect, and, therefore, the procedure would be useless. Independent of that decision, chemotherapy would normally precede the radiation therapy. The radiation would be to the breast and not, necessariliy the lymph nodes. So, to reiterate the standard order would be 1. lymph node dissection (if necessary), 2. chemotherapy, 3. radition therapy, and 4. tamoxifen.
Whether to do axillary dissection when the sentinal node is positve is controversial, but I'd say the majority opinion, absent more study, is that it should be done. And usually that would be before starting chemo. There are centers involved in studies of not doing so, and it makes sense especially if only a tiny amount of tumor was found; the theory is that if chemo will work, it will work for tiny amounts of tumor that might be in other non-removed nodes. The other side of the argument is that it's best to remove all of the tumor that can be removed before treatment. Since sentinal node surgery is fairly young, we don't have long-term answers yet.