both are good processors.
however, the pentium D will outperform the Pentium 4.
the other guy is right, dBTelos; the PD is a better choice for cost versus performance.
if you are trying to build a cheap computer but will still run apps, get the Celeron D for 40 bucks. otherwise the Pentium 4 and the Pentium D, early class competition will hit a hundred bucks.
so you got the Pentium D, featuring two cores, or the P4 with only one core at the same price. Prescott model of the P4, stay away from, that was 2003, when enthusiasts were willing to let the heat issue not worry them a whole lot. It's 2006? Get the Pentium D, it's latter models are cooler in temp.
conroe, yeah, but that's 200 bucks for a CPU. if you just want a running system, you can get a CPU for a hundred bucks and spend 50 bucks on a reliable running mobo and the other 50 bucks on maybe 768 megs of RAM.
the Pentium D is not becoming too far outdated yet. the PD is only taking one step down the stairs as the conroe is readying to take over for Intel's primary performance chip.