there's already multi-threading in things (just not for the features people want it on) and yes going 64-bit would alleviate the main bulk of memory issues once code is fixed up to cope fully with 64-bit pointers.
but the only real issue with being 32-bit as-is for memory is that for very large libraries / lots of artwork, we just run out of continuous memory blocks for things - which going all 64-bit would resolve, but only because there's a lot more 'empty' space which can be allocated, it still wouldn't help with the obvious memory fragmentation issue within the process.
and i'm only talking about memory at the moment since it's an obvious metric people can understand and being a bit more sensible about it's usage with how Winamp already is can have benefits (performance and stability). just like was already started with the 5.64-5.66x releases for the non-Cloud parts as better memory handling of xml files gave us a decent reduction in loading time for modern skins on what i was able to test.
plus if it's seen to be lighter again on such things, then maybe (i know it's unlikely) we'll see people who keep using the old versions will consider and try upgrading to a newer version (as without people using newer versions when they do come out, there's no point in having a dev team as what's the point in paying people to work on something that isn't being used).
and despite what is now possible with hardware, small and tight implementations are what should be strived for (within reason) as we've seen with continued XP usage and so on
anyway, i'm rambling now and not coding, so i'll shut up now.