Essentially a huge MP3 PLayer
 

OK well my friend recently ran into a couple of free monitors and he was going to pawn them off. But i said well u don't have a stereo system in ur basment, so why not just build essentialy a MP3 player from the free monitor.

My question becomes, what would be the cheapest way to build something that only really needs good sound, and to not be extremely slow.

rememeber doesn't need state of the art GPU
the most expensive thing is going to be the OS, i think

If you're starting completely from scratch, with no other spare parts sitting around, it might be most cost effective to just buy a used rig off of eBay. I'd imagine something like a Pentium II or III, around 500 mhz with 256 megs of RAM (maybe 512) will suit your needs. I took a glance on eBay, and you should be able to find something for well under $100. After that, you might want to invest in a better soundcard, depending on how you want to hook it up to your stereo.

And if you think the OS is the biggest cost concern, this might be a good time to mess around with Linux.

I agree with the linux route. The CPU requirements are lower than trying to run a windows OS. Personally I like Mandriva with it nice simple installation. It's hard to beat "free" :)

Er... CPU requirements? Linux?

In the past I used an olde pentium (one!) 133 Mhz, 64 meg laptop as an mp3 player. A p2 or p3 with 256/512 megs is totally overkill, so trying Linux just because it'll run is like... er... well, not needed. :)

Just get a computer and OS that'll start up quickly. You could try older Windows versions or Linux if you want to.