The Core 2 duo is a dual core CPU essentially. The Pentium D has 2 cores on 2 dies as does the Athlon64x2, these are known as MCP or MCM( Multi Chip Modules. The Pentium Dual Core is based on the Core 2 architecture but does not perform as well. The Intel Core architecture was superior to any other desktop processors at that time.
EDIT: Athlon 64x2 is atcually 2 cores on one die..