Mac Mini is a good suggestion. Also, any mini-atx form factor computer with an upgraded hard drive would offer the same functionality. I built an HTPC that only runs XBMC (look up some screenshots, it's awesome) and it handles everything for me. I built mine for sub $300 and use it to stream music from network attached storage, but for another $200 I could have just as easily added 2TB of storage and never have to worry about upgrading the hard drive again.
If you're not into DIY, I'm sure you have a techie friend that would be more than willing to put something together for you for less than what you would be spending on some pre-assembled system from Yamaha.
If you're not into DIY, I'm sure you have a techie friend that would be more than willing to put something together for you for less than what you would be spending on some pre-assembled system from Yamaha.