Motherboard always comes first. If you get a CPU with a faster FSB than your motherboard supports, it's not going to seem any faster except for the stock clock spead.
Example: If you took this Foxconn Mobo
and dropped an AMD Athlon in, it would work fine because the FSB of the Athlon is 1000 MHz FSB. But say you dropped in a Phenom II 940. Well it would be faster than an Athlon because of the stock clock, but because of the limiting FSB factor on the 740G chipset, the higher FSB of the Phenom would be of no use to you.
However, say you go with the ASUS 780V
. So say if you drop in an Athlon processor with an FSB 1000 MHz, you would get all 1000 MHz. Then if you dropped in a Phenom II 940 later on, you would get all of the FSB from the Phenom unlike if you grabbed a cheaper 740G earlier.
I hope this wasn't to confusing.