Place frozen spinach in a 10-cup shallow microwave-safe casserole; sprinkle with instant chicken broth; cover with wax paper. Defrost in microwave oven ten minutes, rotating dish and stirring once; drain well; stir in sour cream and cayenne; arrange artichoke hearts and crab atop; dot with butter and cover again with wax paper. Microwave on high 5 minutes, or until heated through.