Rebate Monger is exactly right, according to tests I've seen. An external drive connected as if it is a plain normal SATA drive will be the same speed as an internal SATA, and he's got the speed right or it might be a little slower than he says, but certainly faster than USB.
A true eSATA connection with either a built-into-mobo controller or an add-on PCI eSATA controller gives you all the proper features of eSATA. But many drives (or external cases) come with an adapter plate that allows you to hook into a normal internal SATA port and "convert" it to an eSATA connector on the back of your machine. You may or may not get all the eSATA functions (like hot swapping), depending on your SATA controller. But if you don't need them no problem.
I got an AZIO external case and mounted in it a SATA II Seagate 500 GB HDD. It has available both USB2 and eSATA connections to the computer. It came with one of those adapter brackets, but since my mobo has an eSATA controller and rear-acccessible port, I am hooked to that. Works like a charm. I can even turn the external drive on and off (has own power supply and switch, of course) while running and Windows recognizes when it is present or not. Do NOT do this during data transfers!
By the way, small detail. I have a different ASUS mobo. On the eSATA port system, the manual spends a lot of time telling you how to add drivers and set it up as part of a RAID array system. But reading it carefully shows that, if you choose not to go RAID, you can simply set it in BIOS to be a plain eSATA port that pretends it is a PATA controller and behaves like any other "normal" HDD.
