Just for the record, the memory bus isn't at fault because it was 100Mhz in the old ones and in the new 550 (the 667 has 133...). It is the smaller processor level 2 cache, which on the old PBs was 1 MB and now 256 I believe.... Anyway, here is an interesting article about this:
http://www.xlr8yourmac.com/systems/Powe ... html#logic
This doesn't solve the problem, I know, but it proves that other people have the same issues....
Also beware of OSX:
There are no sound hardware drivers yet excetp M-Audio's, and Logic is not yet compatible...as far as I know Emagic, this can still take ages.