New To Mac. Need Help, newbie looking for info on used computer Options

i have many questions here. please bear with me.

i am a Windows user currently. i plan on, in the next few months, purchasing a Mac and getting rid of some of my music hardware. complete upgrade.

here is my plan: i am shopping around on ebay for a used Mac G4. hopefully 500Mhz. dual processors would be ideal, but more expensive. hard drive size not a big deal as i can upgrade/add new drives easily. memory, hmm... hoping for nothing lower than 256MB. 512MB would be nice, and more would be better.

what are opinions on operating systems? os 9? os X?
if the computer i buy has no system installed, how expensive/difficult will it be to install one?

is processor speed a big deal? what if i can only find 350Mhz? and what about memory? how expensive/easy is that to purchase and install?

and i just read a thread on hard drive speed. any comments on that? is it that important for music production?

once i get this computer, i plan on using MOTU Digital Performer with various plug-ins and left-over hardware. and then i wonder: what is VST? then i will need a sound card, which brings up another question:

where do i find info on sound cards? any suggestions? is usb the way to go? firewire? what is "audiowire"? can i get a card that is internal, so i don't have to mess with usb/firewire?

any info is appreciated. thanks for your patience.
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