The fastest of the Intel® Pentium® D processors was the Intel Pentium D 965 Extreme Edition 3.73GHz. When we released the Intel Core™ 2 Duo processors I was ready to buy a new system so I went looking for benchmarks to see how they stacked up. With my budget at the time I could pick up the Intel Core 2 Duo E6400 at 2.13GHz and I compared it against the Intel Pentium D 965 Extreme Edition here on Tom's they had 20 benchmarks and I was able to check them. Out of the 20 benchmarks the Intel Core 2 Duo E6400 won 16 of them and only lost by a small amount on the other 4.
So without a question upgrade to the Intel Core 2 Duo E6300.