I don't know what the specs of your computer are but if you've got a fairly decent PC why not just stick some large HDDs in for your media and then stream with your PS3, that's what I do.
Depends on whether or not you want access to it from other parts of the house. I have a server running that I can access throughout the house - but when I get my HTPC setup it'll be also serving as a file/media server so it will be left on.