1. The Pentium D's are more powerfull.
Hyper-Threading Technology allows processors to function more efficiently when running multiple tasks, however a dual-core processor has a second physical core with its own set of dedicated execution resources. Therefore everything else being equal Dual-core processors offer greater performance than single-core processors with Hyper-Threading enabled.
2. Yes both cores run at 3 GHz and Yes each core has its own L2 cache ram. 2x1MB for the 8 series and 2x2MB for the 9 series