You've got to connect your eSata port to corresponding SATA port / eSATA port on your PC, or motherboard. If you have a PCI card based eSata port on your PC, then your max transfer rate will be limited by that of your PCI card. The best solution is to get an eSATA port that is connected directly to the motherboard, and not through an additional card. I think that PCI-Express supports the full speed of eSATA though, so maybe get a PCI Express card based eSATA port, if your motherboard doesn't already have it's own eSATA port.
Also, remember that the HD inside your casing needs to be SATA compatible. You get casings that have an eSATA port but will also accept older drives that are not SATA compliant. You will the be limited by the max transfer rate of your HD, which will be way less than SATA speeds.