My understanding is that two USB 2.0 ports will supply exactly 5W, but that 1394a will be faster for data transfer.
True on the 5W, but not necessarily so on the data transfer. I have found USB to be faster than FireWire on higher performance systems for example. See the section titled "Performance Myths and Realities" in URPCs 19th p. 785 for more detailed information.
Bottom line: USB and FireWire are relatively equal under most circumstances, however either one may be up to 25% faster than the other depending on the specific hardware involved. If you want transfer rates that are 3 or more times faster than either of those, use eSATA or USB 3.0 instead. You could also install a 2nd SATA drive internally using a Ultrabay Slim SATA drive bay adapter
, which would give you the highest transfer rates possible.
Will the 4-pin 1394a port on my T61 supply enough power for the drive to spin-up?
No. The 4-pin FireWire connector does not supply any power, that would require a 6-pin port instead. Scott.