so that will let me access the 3GBp/s speed?
I think its 3Gb/s (gigabits as opposed to gigabytes) and it depends on your HDD. I THINK if your HDD supports SATA II then yeah you will get 3Gb/s on the interface level (I know the Silicon Image chip supports SATA II.)
However your realistic transfer speed will depend on your hard drive and will not saturate the bandwidth of the eSata controller by far. I dunno expect ~50-60MB/s (on average for a large file transfer) at most? I'm not really sure how it will turn out.
Going off at a tangent here but the SI chipset used to implement the extra SATA ports is pretty naff. Running a RAID0 off them I couldn't get a read throughput > 100MB/s whilst using the nforce4 solution I got considerably higher. It won't really affect your eSata - external storage situation but I thought I'd throw some random info around