I usually had problems with crackles with ASIO and sonar 2.
my system:pulsar1+pulsar SRB+luna with ASUS p4b533 with 2.4GHz, 1MB ram.
yesterday i made two changes:
1) in the bios i setted the "CPU/RAM frequency ratio" (or similar) to 1:1 . previously it was set to "auto", and i think this meant a ratio of 4:3
2) i changed the disposition of the cooling fans. now i have a fan that blows air directly to the creamware boards, to cool down a little the sharks. then, i had a fan designed to be mounted in a PCI slot and supposed to bring out the warm air. i mounted this fan on the top of my case, directly above the CPU, and now this fan extract a bit of the cool air that the CPU produces.
i think these two steps were a major improvement in reducing the ASIO crackling