I have a PCIe card for my eSATA with a 1tb external (phntom drive, green drive, it's a WD green drive). Performance is fantastic. Hot swappable. It's like having the drive internally.
I got this device: http://www.newegg.com/Product/Prod
It was $20 with free shipping.
(Did it for the same reason you did; files too large for a network to handle quickly, via NAS)