The Smithfield core is essentially two Prescott cores stuck together on a single chip. No other major architectural differences exist, except that the Pentium D line does not have HyperThreading on it. Unlike the Extreme Edition 800 series which can have a total of four logical cores (two physical cores with two logical, HyperThreading cores each) the Pentium D is strictly two single threaded cores.

The advantages of having a true dual core processor over a single core, HyperThreading processor are fairly noticeable and users should see a good sized increase in performance going from a HyperThreaded CPU to a dual core CPU in benchmarks and applications that take advantage of multiple processing threads. We’ll see this illustrated in several of the coming benchmarks.

http://www.pcper.com/article.php?aid=145&type=expert

No overclocking info