A. For NT Workstation on Intel 12MB is the minimum, however 16MB is the recommended min, 24MB will reduce virtual memory usage and increase performance. For RISC based processors 24MB is recommended, and 32MB to improve performance. Most NT people will say the real acceptable performance numbers are 40MB for NT Workstation and 64MB for NT Server. It really does depend on what you will be running on the server.
For NT Server 16MB is the minimum, however most sites have 32MB.