Well, by and large the decoding is done by the CPU. A lot of soundcards will have DSPs on them for superior quality and speed of conversion, but it's a matter of whether drivers exist for the Amiga to be able to use them. And as regards PCI soundcards, there are none that I'm aware of. So you may buy a card with a DSP but that's only gonna sit there and do mothing while your CPU is doing the decoding.
On a side note, Paula is actually quite good, and will use less CPU time when written to directly as there's no AHI involved, but there's not a whole lot of difference there.
