Usually it means a comptuer company like DELL or Gateway. The idea is that a manufactuer does not need a flashy box or glitzy manual or extra software with the hardware they purchase. So usually it's put in a white/brown box wrapped in anti-static plastic with a driver disc, then sold at a cheaper rate. Also sometimes, the quality of the device is lower, such as a video card, it may be clocked lower or a CPU which may not overclock as well as a retail counterpart(OEM CPUs don't come with a heatsink and a fan either).
Also OEM parts could be classified as any part that could be used in building a new computer.

As for SATA, it's great in all. But we mechnical limitations with hard drives. Most consumer drives can't even max out ATA/66. Waiting around for a drive that will no doubt be MUCH more costly and not produce any better results then current drives would seem foolish. :\