Too bad signing in is currently available from 4 countries only.
The country restriction only applies to the Xbox apps: Music, Video, Xbox Games, Xbox Companion.
All other apps support signing in in any supported country/region, with the partial exception of Xbox Live-enabled games which only support signing in for regions that are supported by Xbox Live (which is a larger list than the Xbox apps). These games still work in unsupported areas but achievements and other integration features will of course be unavailable.
About GPU compatibility: nVidia's latest drivers work perfectly on 8 (there are official Win 8 versions of the drivers (the same as with 7 and Vista). AMD have released preview drivers, don't know how they work.
I am seeing quite a few problems with IE10. It runs faster than IE9, that is certain, but some interface issues come up from time to time (not related to rendering websites).
Nvidia's preview drivers work with all currently supported cards but AMD's only support a certain subset of cards. If you have an AMD card check whether it's on the supported models list
before downloading. If your card isn't supported use the driver from Windows Update instead; this is fine for everyday use but some games won't render correctly.
As for IE10 are you talking about the desktop version or the Metro version? The Metro one is completely GPU accelerated (rather than just the page within it) so visual issues with it could be due to the preliminary nature of both vendors' Windows 8 drivers rather than something inherent to the app itself. Both vendors will probably have finalised Windows 8 drivers within a couple of months.