I just bought the Asus add-on card so I could get SATA 6 on my old board. It has 4 PCIe slots so I thought it would be something good to get in addition to my new 600 gig Raptor. Everything is installed and working fine but my problem/question is with the bus speeds. Since I am also running two video cards @ x16 it knocks down the other two slots to x1. (which I didn't realize till after the fact) I'm getting transfer rates in HD tune at around 140-150 MBs which I assume is pretty good. Is there anything I can do to change the bus speeds or should I even bother?
140-150 is not far off what the performance of that hard drive should be, as the absolute peak I've experienced is in the 170's, but in actual, everyday use, it is right where the speeds you've experienced. A PCIe x1 has enough throughput that a single hard drive will not exceed that throughput. Now if you attach another drive and/or attach a couple USB 3.0 devices and you'll take a hit. I suggest using the second SATA 6 for a DVD/Blu-Ray drive to open up a free SATA3 port on your motherboard for other HDs; this will alleviate your issues.
If you want to know without much hassle, plug it in to a SATA3 port on your motherboard and you'll quickly see that SATA3 can offer the same performance as the SATA6 for that drive. Only a RAID 0 array of platter-based drives, or current SSDs, would exceed the max throughput of 250MB/s (more or less...) for SATA3, and necessitate a SATA6 controller.