try Iori's SPU plugin, and the internal SPU core, as well.
I use Iori's and I get great in-game music play (ok the music itself isn't that great

) but still have some problems with MDEC. I watched it in VGS anyways.
Also I'm getting perfect frame rate (60fps) most of the time, with Lewpy's glide 1.33 @1024x768
my sys spec is in my profile.
More RAM does help, I used to have 128mb and still noticed improvements after upgrading to 384mb. Don't know why exactly, perhaps the emu (+ all the plugins?) could just use that much RAM.