X-bit Labs has written a comparison of five 640GB hard drives, two from Seagate and three from Western Digital. You can read it over here.
Dual-platter HDDs have always had an ambiguous standing. They are not remembered as “the world’s first to achieve the capacity of so many gigabytes” but still enjoy well-deserved popularity. It is dual-platter drives that used to have the lowest data storage cost per gigabyte. They have only been outdone recently after the considerable price cut on 1-terabyte models. You can also expect very good performance from them: each of the five HDDs included into this review is based on full-size 320GB platters whereas there can be drives with lower recoding density among 500GB and 750GB models. Well, it is going to be interesting to compare them!