Would a dedicated gfx card take the worry away?
http://www.tekheads.co.uk/s/product?product=610145
I dunno
Depends on the content, I expect that for HD/Blueray then the graphics card will (Not sure about the card above), and correctly encoded AVIs also, but some AVIs are not encoded in a way that the chipset recognises.
I have one of the 780G ASUS AMD boards and havent had any problems, but on some "h.264" AVIs get high CPU usage, others I do not, but nothing that causes any issues in playback.
(ASUS M3A78-EMH HDMI 780G, Athlon64 X2 @2.5Ghz, 2GB RAM) - My setup with on-board graphics - rest of setup in sig, very quiet and very happy