I bought the Asus Eee PC Seashell Series 1015BX with HDMI output. Windows 7 Starter was pre-installed. I added XMBC, plugged it into the Onkyo and we were away.
Music playback sounds great. XBMC seems to produce better sound than iTunes or WMP.
I downloaded 720 and 1080 video samples from the Geekzone forum. Playback was smooth with comparable/better quality to HD-DVD.
Overall I'm very happy, for $450 the little Eee is cheap and easy way to tap into digital media.
Next I'll add NAS and some sort of remote control.