You could have it as an external drive, though I wouldnt see much of a benefit, unless of course you actually need it to be moved around to another PC etc.
Windows will generally preserve the drive letter as long as you keep the boot conditions the same, just keep it plugged in at startup.
Just a suggestion, you could get a pair or more of disk drives and raid them instead. Would be quite a bit easier on the wallet. While still having enough performance to handle things.