I have two HD's, a raptor 36GB 10k rpm, and just a regular hitachi IDE drive. I burned with both, even when burning a file from the raptor, it won't go past 1.8x. I'm using decryptor, Nero, manually set the speed to 2, 2.4, 4, and it never goes past 1.8x. I've also defragged many times using O&O defrag.
This is the Intel 915P board, only one IDE channel and 4 SATA channels. I'm running the NEC through the onboard IDE, and my hitachi through a PCI IDE controller.