I don't have a N800 yet, but from what I've heard, ITOS 2007 is not quite stable yet. Maybe it'll take some firmware releases from Nokia to get this near the stability of the current ITOS 2006. The first few firmware releases for the 770 used to be quite unstable and slow, too.

And considering YouTube, there is hope. Even the 770 can play many YouTube videos smoothly with mplayer, so the necessary CPU power is definitely there in the N800. My guess is that the Flash 7 plugin is not optimized for the N800. Hopefully, this'll change with future firmware releases.