Kobatoislove :
The Pentium D Processor is the more preferrable one.
The Pentium D has two cores compared to the Pentium 4 which has only one.
Even if you take Single Threaded Applications into account.
The Pentium D will still rock because many of the Processor Resources can be shared by the two processors giving the computer a headroom for Single Threaded Applications.
I wonder why you would want to buy a processor that is 11 year old.
Thank You for the reasonable reply, I'm gonna be going with the Pentium D.
The reason I'm getting such a old processor, I'm building a Very Old Computer build for Windows XP for older Application.