the only problem i could see is directX.
linux and mac os currently run a hacked out version.
however this may not be a problem if they pull Nvidia onboard as a partner. As i do believe Nvidia own 3dfx glide. This would be a bit of a programming hash up to get it back up to current specs. However it could have the bonus knock on effect of finally allowing linux and mac os the opportunity to compete in gaming. Open GL could also be used, But i would be under the impression that valve alone could not manage the workload to get the API to function without breaking copyrights, It would be down to a Gfx developer to make a solution.
Furthermore it could see a lock out of games for windows. Since a free program that can cope with directx style information, Either under emulation or with a decode would in effect allow developers a choice they have been denied for a long time.
PC specs for gaming don't need to be super hot, Provided you have adequate specs to run at 30fps, and enough graphical punch to use all the tricks then you can use a pretty basic model. Unlike a pc you won't have people comparing numbers overclocking. So the system can probably cope on a dual core mATX style system, with a sub £100 gfx card. Forget about the high end stuff as they simply will not need it. This means a sub £250 system should be viable. Steam could buff this by reducing your graphics options, and optimising for 30fps.
I would expect it to be running something along the lines of a sandybridge g620, mATX, 2Gb ram, Gtx 560, and a budget green hard disc. You can build this sort of configuration for £250-300 currently at store prices. And it would play most games at 30FPS, at decent settings. That's without having bulk discounts and optimisation in the mix.
I believe the deciding factor will be directX overall, as i can't see M$ giving up it's software to a competitor, especially when it could be damaging to xbox and windows sales in general.