It's a bit of a lottery. Some people have never managed to solve the X-Fi 4GB problems. I think we're in the minority but a significant minority. These range from unpleasant noises with 4GB RAM to my problem, which is no recogniseable sound apart from the odd click or pop. It's completely unuseable.
I've a feeling it's not as simple as which motherboard you use with an X-fi. I spoke online to a guy with an X-Fi XtremeGamer and the same motherboard as me. He could run with 4GB RAM fine. We were using same BIOS version, same settings, same drivers. Decided I was tired of trying to troubleshoot it. It could be that it's some other problem but I'm tired of trying to troubleshoot it.
I've even wondered if there's some variation in X-Fi hardware (or if mine is faulty). I do suspect that if I tried another X-Fi it might be fine.