Many of the Pentium 4s have Hyperthreadting (HT) technology, which makes Windows operate more or less as if there were two CPUs (one physical, one logical). This helps grearly with multitasking, but it is not the same thing as having two physical cores.
The 800 and 900 series Intel processors do have two physical cores.
To follow on from Taeric's excellent answer.
Pentium 4's with Hyper Threading have a single physical core. HyperThreading fools the operating system (if supported) into thinking that the CPU contains two CPU cores (one logical and the other physical) so that multiple threads can be executed in conjunction. This will increase performance in many cases and really does help with "real world" tasks. However it can be less in some other applications, a lot of games for example.
As mentioned the Intel 800/900 series contain two physical cores, the Extreme Edition versions also contain HT Technology, so four cores
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