Celeron 847 will run around 85-90% CPU streaming Netflix HD. So, it will do it but it gets pretty warm. (This was my experience testing the desktop form of the 847).
1080p file played through WMC, the CPU was under 20% and things ran cool and quiet.
XBMC's internal renderer in Windows was a no-go. You get macroblocking with DXVA on and the CPU couldn't handle it with it turned off. If you're willing to launch an external player though it goes back to the 2nd bullet above.