We returned for the appointment this morning, and actually left it feeling a little comforted after talking to the vet after the surgery.  She said yes, surgery is a little risky for an older dog with a heart condition, but that they take every precaution and tweak the meds and anesthesia for each case individually, and she works with the cardiologists and anesthesiologists to make it as safe as possible.  Plus, she seemed more concerned with the after surgery stuff (whether she got all enough of the tumor if it turns out to be cancerous, and the fact that the incision will be at a bad place) that with the surgery itself.  That made us feel a lot better (or at least, less frightened) about the surgery itself.  The surgery is scheduled for Monday morning.