Compatible processor for dual ddr400

I bought a second hand desktop, and the seller told me that the processor is a dual core. but when i reformat the computer i found out that the pc is only running on Pentium (R) 4 and when i check the board the memory is dual ddr 400 and 512 mb only.

I am confuse to the classification of the processor, how will i know if the processor is a dual core and what type of memory should i be use?

Is pentium (R) 4 can be classified as dual core???