Let's not spin the "shared IRQ" myth yet again, shall we? The reason why moving the card into another slot magically helps is, it gets you a clean driver reinstallation. That's all, no more no less. And it works only once per slot, because Windows remembers.
Specifically to this problem here: The X-Fi cards crackle and pop because they're not getting as much BANDWIDTH as their designers would have liked them to have. This isn't even something to do with IRQs. And no, "DMA resources" don't even exist on PCI. DMA on PCI is exclusively bus master DMA, which means there are no resources of a central DMA controller.
So, AllGamer, please face the facts and finally drop out of ISA mode. Thanks.
(And if this all sounds familiar, that's because you've been told the same thing in at least two other forums
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Now, what to do here? Well, nothing really. The X-Fi cards need /perfect/ PCI bus latency and throughput, something which just doesn't exist in a realistic system. So short of removing other bandwidth hogs like USB webcams or PCI storage controllers, things won't improve.