The single or dual processor matters much less than platter density. Newer hard drives use 500GB platters ( 1 x 500GB, 2 x 1TB ect. ). The best bang for your buck drive has been the Samsung Spinpoint F3 for a while now.
If you want a real speed boost from a mechanical drive then you have to increase spindle speed and go to a 10,000 RPM drive such as the Velociraptor.
The fastest drives are SSDs or solid state drives. These have no moving parts to limit their speed. They are still quite expensive compared to mechanical drives though.
I have always had fast hard drives but the only real difference I saw was the upgrade to my SSD.