Firstly, there's a huge difference when you say Dual Core and Pentium D. There's a series of cpu called Core2Duo, which is newer and much, much, faster than the older Pentium Ds. A 1.8GHz C2D can take on a 3GHz Pentium D in some cases, while taking less power.
Pentium D belongs to the Pentium family while the Pentium Dual-core belongs to the C2D family. There are newer C2Ds out now, so the E4600 is rather outdated.
And yes, the 4400+ is better than the Pentium D 925.
Yes, Pentium D is a dual-core, meaning there's 2 cores.
The name Pentium Dual-Core is also a dual core cpu, but from the C2D family. Intel just decided to make it confusing to us consumers. There is a huge difference between Pentium D and Pentium Dual-Core (Dual-Core as in the name).
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