The original Raptors are pretty much superseeded in performance by the high-density 5400/7200rpm disks of today, the Velociraptor does add performance benefits though; its the fastest consumer-class HDD.
The cache is not important here; its about data density, firmware and rpm.
Your 640GB is likely a 320GB-platter generation drive. Though current drives already have 500GB platters, this still falls into the category of high-density drives. So all-in-all i wouldn't say the Raptor is much faster, it may boot faster but will be slower in other areas so about the same i would say.