OK, here's what I found out about MT/S (I know what FSB is):
MT/sec
(MegaTransfers per SECond) A measurement of bus and channel speed in millions of "effective" cycles per second. MT/sec is a rating of the actual, delivered speed rather than the speed of the interface clock. For example, if timing is derived from both the rising and falling edges of a clock cycle rather than one complete cycle, the effective rate is doubled: a 400 megahertz (MHz) clock yields 800 megatransfers per second (MT/sec). In this case, using 800 megahertz (800MHz) as a rating would be technically incorrect because it is not the true speed of the clock.
So, if I understand correctly, the faster board would be the 800FSB. Correct?