Pentium D is a dual core processor, Pentium 4 is a single core processor

Pentium D lacks HT unless you purchase the EE series, while Pentium 4 has HT (2 logical processors: 1 physical, while Pentium D is 1 logical:1physical, but 2 physical processors total, and Pentium D EE is HT enabled, so it shows up as 4 processors)

the Pentium D will perform better for video editing and most likely audio work, while Pentium 4 will be slightly behind
in gaming they aren't much apart, as games don't really utilize dual cores yet

Pentium D has (currently) a higher heat output, and costs more, while Pentium 4 lacks the second core