Both 754 and 939 have an 800 Mhz FSB. 1600 Mhz is just a comparison. You can read about it here
When they say it can technically reach 2000 Mhz, that is because the FSB can be clocked to 1000 Mhz (250x4) while keeping stability*. Considering the CPU will be processing 64 bits per clock cycle, it will be sending twice the data at 1000 Mhz (250x4). Hence, 2000 Mhz (when comparing to 32-bit).
It's the same with an 800 Mhz FSB. The CPU will be sending twice the amount of data at 800 Mhz, therefore it is like 1600 Mhz when comparing to 32-bit.
*You need sufficient cooling (on CPU and NB), high performance memory and a quality motherboard.