Dual core processor has 2 distinct processor cores inside one chip. AMD's X2 and Opterons (Denmark, Italy, Egypt, Sta Rosa and Sta Ana cores) Intel's Pentium D, Pentium EE, Core Duo, Core 2 Duo (Conroe, Allendale) and Xeon (Paxville, Tulsa, Conroe, Dempsey and Woodcrest cores) are ALL DUAL CORES.
Core2 Duo is specifically a new technology, high performance processor that does more per clock cycle and at lower wattage.
Cheapest version= Pentium E2140 and E2160.
Still cheap but better= E4300, E4400, E4500 (Allendale)
Fast Core2 Duo = E6xxx series
Awesome = Core2 Extreme X6800