Im using a GF4 mx 420 which isnt officialy supported however is working fine for almost everything, the only thing i have a problem with is playing MP4 files, which always gives an error message (which i have confirmed is due to the cards crapness), TV, Recorded TV, DVD's, AVI, WMV all play perfectly from with MCE however, this is with the MCE2005 edition.
MP4 plays perfectly on the same system in media player though.