Is Core 2 Duo that much better than Pentium D?

First, I recommend you read this article which will put a lot in perspective for you:
http://www.tomshardware.com/2006/07/14/core2_duo_knocks_out_athlon_64/

Dual-core CPU's are able to handle larger workloads without sacrificing as much speed as a single-core processor. In other situations involving light workloads, the advantage isn't all that much. So between the Pentium D and the Pentium 4, it just depends.

The Core 2 Duo, on the other hand, isn't just another dual-core CPU. Unlike the P4 and PD, the Duo is redesigned from the ground up (partially modeled after the Pentium III and Pentium M). A 1.6GHz Duo for example can outperform a 2.8GHz P4 or PD with ease. Because of its lower clock speed, it's also able to run a lot cooler. Pentium D's are notorious for running very hot, even the newer models that were reduced from 130 Watts to 95 Watts.

There quite a few articles you can sift through here:
http://www.tomshardware.com/cpu/index.html


If you don't plan on turning your PC into a movie editing studio or intense gaming rig, then I'd say go for a cheap P4 (or better yet an Athlon 64) unless you can afford the Duo. Stay away from the Pentium D at this point.