The $500 Steam machines that I saw announced a week or so ago are still a little hazy with them just saying things like "a multicore AMD processor". That is extremely vague and there are a ton of good and bad AMD CPUs that could fit such a description.
If you are not comfortable building your own PC then the Steam machine, even after adding about $100 for a copy of windows, still sounds like it could be a decent prebuilt computer.
Have you accepted Raspberyl as your loli and savior?
Something like that would be good. You could probably get it closer to $600 by picking a different case, installing the OS via USB instead of a disc to remove the disc drive and going for a 500W power supply. You could also wait and try and pick up a 7870GHz edition graphics card for $150 but they are out of stock at the moment. Something like that for around $600 will be a solid PC.