No, your drives are not rated at 300 Mb/s, the SATA II interface is rated at 300 Mb/s. A drive can only read data at the speed the data passes under the head. 300 Mb/s is the burst speed from the drives cache. Also, if the I/F on the motherboard is PCI, that is limited to a max of 133 Mb/s, (unless you are using Server motherboard with 66 MHz PCI bus and Server grade PCI controller) as long as no other data transfers occur. 300 Mb/s is just a marketing ploy to get you to upgrade. Use HDTune if you want to test your HD speed.
Agreed, also the write speed is usually slower than the read speed. So at a minimum, the transfer rate will be as slow as the write/read speed of the slowest drive, not to mention any other bottlenecks/overhead that might be caused by your configuration or OS. I don't know if your drives are included but this may give you some idea.
No, the SATA 300Gb/sec. number is just the bandwidth of the interface. It has no bearing on the drive speed. The drive speed is limited by the physical characteristics of the hard drive itself, they haven't technically saturated the speed of the PATA bandwidth yet, and certainly not the SATA 150Gb/sec. Your Raptor is SATA 150Gb/sec.
Look at the charts that I have linked and check the various benchmarks. Also it depends on how fragmented your drives are and whether you are transferring one large file or many small ones, there are a lot of variables.
Here is a link to HD Tune that mike99 suggested. You will not be able to bench the write times because the drive is not blank but it will give you some idea. I reiterate there are also other variables that will slow you down.
I did a quick google. Most responses where about the network slowness, but one person said they had issues with anti-virus 'causing HD to HD slowness similar to yours. Disable any anti-virus, reboot, and give it a shot.