Ahhhhh, i am not good explaining this, but here's a little info (to the experts: please correct me if i'm wrong):
You mentioned 2 GHz. That would be the speed of the HT (Hypertransport) not FSB.
A64's do not have a FSB; they have what's called HTT.
To get HT bus speed you have LDT*HTT*2 = HT bus speed
E.g on my system:
LDT @ 4x
HTT @ 240
= 960*2 = 1920 MHz HT bus
Now RAM normally runs the same speed as the HTT (200:200)
You can use a divider though; allows you to run the HTT higher than the RAM.
E.g. on my system:
Using 166 RAM divider aka 6:5, i run my HTT at 240, which puts the RAM at 200
I would recommend researching the subject, since i have major trouble explaining it simply.
I hope someone with better explanation skills can reiterate & make it more understandable...