It's a moot point having a fast HD if the heads are having to madly seek all over the place... you can count the days before it fails dramatically.

BTW even though the SATA 1 interface can push through 1.5Gbps the drive itself most likely cannot.
If you're serving a limited set of content (say only a gig or two) then a file system in RAM would be one option to consider.
Personally I would go for the multiple server option, if you're pushing out that rate of data then it's probably something reasonably important; what happens when the server fails?