Looking for enough cores to start seeing complex behavior? Check out this Intel Core 2 Quad Q6600 Acer Aspire from CompUSA for $750 (sheesh, the chip goes for about $580!).

Or, if you're rolling in the dough, CNet quite likes the HP Pavilion Media Center TV m8120n, which will set you back ~$1200. It comes with a cheap graphics card, but sounds like a good media server.