I know the archos pretty well and have a 770. they are vastly different devices. the other threads may talk about the differences (but could take long to look thru).
Archos is great for watching video - much better than N8x0, although N8x0s do a good job too. But there are a million things you can do on a N8x0 that you can't do on an archos. I think the N8x0 are better for internet use (but check out the archos experience yourself in threads of other forums). Plus, you can run a million apps, email client, newsreader, games, ebook reader, on and on and on. See the thread something like "what do you have on your N800" to see what people use it for.