$500 video card, huh? After complaining that $500 for the XBone was a little pricey (I bought one anyway), building my own got just a tad less appealing. I will be interested to see how much volume pricing brings down the final price at release.
Anyway, this is just one config for the SteamBox. Each beta tester has a different hardware config I'm pretty sure, from real cheap, up to an absolute beast. They did that because that's the plan with the OEMs too
From my research, you'll be able to get or build an SB yourself cheaper than a next gen console with the same power at least. I'm excited by that!
Nowadays you can go a very long time with even just a medium spec'd gpu and still compare favorably to consoles. But unlike consoles you have the option to upgrade it if you want to.