We presume iBuyPower should be one of the top contenders. They plan to make a Steam Machine around the size of a PS4, and just a bit smaller than an Xbox One, which will include a multi-core AMD Cpu, 500GB hard drive , WiFi, Bluetooth, and an AMD Radeon R9 270 graphics card.
That card alone will set you back $180. If everything pans out then you can get these specifications for about $499 which could pose a major threat to Xbox One sales.