my experience on a 600MHz via x86 CPU is that flac takes much less CPU power to decode (at least a factor of 2) than ogg vorbis. That was using gstreamer codecs.

there may be variations between x86 and arm, but i can't imagine the n800 having any trouble playing flac if it can play vorbis in software. if the decoding can be pushed to the DSP then it would be even better.