The fastest SATA is the WD Velociraptor, which, while noticeably faster than most hard drives, cannot hope to compete with a good SSD in almost any area.
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Reply to cjl
It depends a on what you're looking for. If you want a hard drive that can transfer large blocks of data quickly (ie, copy large files), an SSD is somewhat better than a hard drive at reading, but probably not at writing.
But if you're looking for a drive that can do lots of small random I/Os very fast (which is what's important for booting and starting programs), then an SSD is miles ahead of any hard drive, even hard drives in a RAID array.