Which is better

You might want to be a little more specific in what your're looking for, and how you'd define "better".

again, "better" for what?

Probably the dual core is the "better" chip - more advanced technology which greatly improves multi-tasking and will improve performance on any software optimized for multi-threading. The speed advantage of the single core P4 listed is very small.

The 400 is a desktop, the 200 you pay a premium for a smaller quiet package suitable for the living room entertainment center. Neither give you as much room to grow as the 600, but the 400 is the more expandable of the two. Depends what you're look for - as in what do you want it to do / how are you going to use it?