First we'll look at Windows 7's minimum requirements (as Windows 8 will most likely be the same or a bit lower on average):
onboard graphics with 8mb
10gb hard drive
XBOX 360 specifications:
3-Core 3.2ghz cpu (PowerPC, a little like ARM)
At least 20GB hard drive except for the s 4gb and the Arcade models (those will need a hard drive)
Points that could theoretically allow for this:
Windows 7 can run pretty well on 512mb with some optimizations already
Windows 8 will be slimmed down a lot compared to W7, and unnecessary things can be stripped out of the OS (and made available through the Metro App store in the case that some people will use it)
Metro is similar to the latest XBOX interface enough that it could translate well to gamepad navigation, with some tweaks
Metro apps will work on x86/x64 processors AND arm processors, so Microsoft could possibly make them run on a PowerPC version of Windows 8
The Xbox version of W8 will not have access to desktop mode, which reduces a lot of overhead.
XBOXs currently shut down most of the OS when playing games (so the resources can be given to the game), so W8X could do somewhat the same thing
you should be able to run pretty much any Metro application, providing it does not exceed available resources and W8X has strict background app-suspending protocols.
plugins may be a possibility, but I'd expect them to be only available trough the Metro App store after scrutinization to ensure they won't slow down the system or stuff.