Best pc budget pc for running plex or xbmc?

I'm sure this has been answered before, but I'm looking for the best bang for my buck. I wanted to get an affordable htpc that would run plex and play x.264 blu-ray rips without any problems. I would go with the mac mini, but I can't swallow the $600. I'm currently using a western digital live hub ad a boxee box, but really wanted the ui and capabilities of a htpc. Anything at Best Buy right now that would work for around $300?