Well that thread, as good as it is, doesn't help in terms of the 64's.
Generally lowering the HTT multiplier down one is needed when OC'ing, more specifically when raising the HTT/CPU up from it's original 200.
On the 64's you can lower the multi, but not raise it, so that leaves raising the HTT/CPU as your only option.
The HTT multiplier is what gives you the overall system HTT of 1600 or 2000 depending on your board.
For a 2000 it'd be 5. 200x5 = 1000 and due to the 64bit nature it's then 1000x2 I believe to get your 2000HTT, but this is why the MSIs just label it as 1000 or 800 instead of 1600 or 2000.
So you have 200x(CPU Multiplier for your total clock speed)
And then that 200 is multiplied by the HTT multiplier, if it's 1000HTT it'd be 5x so you'd want to lower it to 4x to give you some headroom for OC'ing the CPU unless you'd want to OC the whole system HTT inwhich you'd leave the HTT multi at 5x and just hope for the best as far as stability goes.
Then regular OC'ing basics come into play. Slowly raise up the HTT/CPU from 200 and when needed raise the Vcore.
If you don't want your RAM being OC'd as well you'll need to underclock it to 333speeds and then when you raise the HTT/CPU past 200 the RAM will eventually raise up near it's original DDR400 speeds.