Current Fastest Mechanical Hard Drives?

Seagate recently announced that they will soon be shipping drives with data densities of 1TB per platter. I don't know if these will spin at 7200 RPM, but it may be worth waiting a little longer. That said, Seagate and WD are having a lot of reliability problems, so maybe neither would be a good choice at the moment.

BTW, Seagate recently purchased Samsung's rotating storage division, and WD bought Hitachi's.

Your title asked about the fastest mechanical hard drives. There are 15 000 rpm drives, but typically not available in SATA (SAS is more common). There are 10 000 rpm drives, most readily available to us in the form of Velociraptor drives. The 7200rpm drives are more mainstream, rather than fastest  

RAID 0 is risky - if you lose either drive then you lose everything (because the data is striped across the drives with no redundancy) - you can't get anything back after a drive failure. And with two drives you double your chance of failure. You will have to make sure you have backups of everything. Then again, you probably want to have backups anyway.

RAID 0 would be fine if you use the drive array as temporary space / work space - it wouldn't matter if the array failed, then. I doubt that's how you plan to use them.

64MB vs 32MB cache won't have a big impact on your speed - it makes more of a difference to a server workload than to a workstation.