Well, the advantages are many. Most notably higher power efficiency (approx. 30W less TDP), and higher IPC leading to much better performance on lower clock frequency. 3GHz Pentium D is easily beaten by even 1.86GHz Core 2 Duo in most tasks. But there were numerous benchmarks on the subject and I am sure you will find the data you need just by visiting few trusted review sites such as tomshardware.com or anandtech.com among the others.
