Perhaps they'll do a tier system like "Steam Machine Bronze / Silver / Gold / Platinum"

It would have to be some sort of rolling release I think, because this year's gold hardware will be next year's silver or bronze.

I'm pretty sure they'll build in the equivalent of the windows performance score that can be sent to Steam's servers, and they'll simply be able to show you the games that will run best on your machine and hide games that simply won't run at all.

You'd need to know before you buy the hardware, though.