Well, Nokia promised new codecs so.. as it is now there's not much I can watch, everything I have and everything I can produce with other devices don't work because of the missing codecs.
At least the CPU should be good enough. My Palm T3 can play almost anything (through the free tcpmp player), and that CPU isn't any faster afaik (and it's years older). The N800 apparently has some throughput problem with the internal bus to the LCD, but as it does work ok (in the latest OS incarnation) with the codecs it has then it shouldn't be any problem with other codecs either: The T3 uses just the CPU for the decoding and if it can do it..
Anyway, that's what I'm thinking. It may be all rubbish, we'll see.. hopefully!