which processor is better Intel Pentium D 965 Extreme Edition Processor or   

I use Intel Pentium Dual Core. So I recommend you this.

Core 2 Duo E8400,without any doubt.

It has lower power consumption and higher IPC (Instructions Per Clock). So don't take care of the "ghz" number.

A Core 2 Duo at 3 ghz imho can outperform a Pentium D at 5ghz.

Hope it helps,bye :)

I just want to let you know a tip; I hope this answer the your question.
Any Intel Micro needs a good configuration in Motherboard and RAM to get the best performance.
The trick is as follow: for example, if you want to put 2 GB in RAM memory, buy 2 modules (1GB and 1GB) this will turn on the Double chanel, double comunication between the RAM -- BUS -- Micro.
This is the trick to get a faster Computer.
I learnt this in a Intel's course last week.

Hey,

I think that the Core 2 Duo is better, mainly because it is advanced in its power saving features. So if you have the Pentium D that is about the same speed (maybe even worse), it will use much more energy. Make sure you have a good day, Happy Lotting!!