If you have a BX board, what processor are you running? If you're not running 900 MHz+, don't even consider wasting money on a GF3. I'd actually suggest against it in any case, but with less than 900 MHz you're not even going to be able to take enough advantage of it to make it worth your while.
Well, they give the system bus as 273 DDR, which would be136.5 SDR, and the AGP bus would then be 68.25 as opposed to the standard 66, which is nothing. If I'm reading all that right, they're getting most of their Mhz out of the multiplier (11).