These CPU's have 4 physical cores. Thery're based on the Nehalem architechture. Also they have HT tech. For each physical CPU core, the OS adds aditional virtual core, and when possible, the OS shares the workload between the physical and the virtual core. In this way the OS sees the CPU as an 8-core CPU, and makes the 4 CPU cores work more efficient.
The i7 CPU's have numerious improvements over the Core 2 CPU's, an integrated DDR3 mem. controler, tri-channel DDR3 RAM, QPI, large L3 cache and the i7-980x (six core CPU using 32nm tech) has Turbo Boost feature.
You need to be more specific about the dual cores you want to compare with.
It depends on the model.
If you are considering desktop CPUs,then like unknown_13 said, they come in quad and hexa core models(which with HT,it shows the double number of cores in task managers)
However for mobile CPUs, i7 has a dual core version too which is i7 620M, and all other models are quad cores.