Hard drives don't run at 3 Gb/s, the SATA interface does. The only data read at high speed is the content of the HD cache (burst rate). The sustained rate depends on the model of drive, but even a velocorapter cannot read data from the platter at 3Gb/s. To measure your drive performance try HdTune or HdTach.
I have not used HdTune, but I've seen other broadly-based benchmark systems that yield an average data transfer rate over almost a whole disk drive in a long series of data moves. Numbers I'm used to for SATAII are around 40 to 60 MB/s, so yours look quite good. Truth is, HDD's can't provide data to keep up with the max burst rate possible on either SATA or SATAII interfaces.