" So... before I start fiddling around inside my PC... can anyone tell me please is my Ultra ATA/100 hard drive 100% backwards compatible with this Ultra ATA/66 PCI Card ? "
Should be fine. Best way is to try it out, it isn't going to hurt anything.
With the computer shut down and the power lead disconnected. open the case and insert the pci card. start the computer and let windows find the card. Install any drivers that came with the card or search the web for drivers, if you need them.
When the card has been recognised, shut down the computer and install the hard drive. connect it up to the pci card and start windows. Windows should find new hard ware and install the hard drive for you.
DMA is direct memory access.
Does what it says on the tin, just uses the memory and not the cpu.
It is advantageous to have it enabled. When reading or writing from the hard drive, if the cpu is not needed, then the information is stored in the memory. This promotes faster reading and writing from the hard drive or cdrom, speeding up the system.