That's an interesting idea... Although I'd have no idea how to set that up. But it'd make a lot of sense.
If you do not want to re-install anything, you can delete all your unnecessary files or put them on a separate partition, to shrink your C:\ partition down to the size of the SSD. Then, you clone the C:\ partition on to the SSD, then manually set up a D:\ boot loader drive for Windows. You use a Windows Recovery live CD to repair the bootloader, and then you are good to go.
USB 3.0 is technically around 4.8Gb/s and SATA 3.0 is like 6.0Gb/s
transferring huge movies the speeds aren't really a huge deal.
Make sure not to confuse bits and bytes.
Anyone else agree on this? (Western Digital 4TB SATA III 7200 RPM 64 MB Cache Bulk/OEM Desktop Hard Drive, Black, WD4003FZEX)
You don't need to be very picky about SATA III support. Any mechanical hard disk is not bottlenecked by even SATA I. So if you can find an older, cheaper drive for the same capacity, get it.
Also, $250 is too much for a 4 TB drive. You can get a Seagate ST4000DM000 4TB 64MB Cache SATA 6.0Gb/s 3.5" HDD for $160.00, on Newegg.