TechnoDoc, I wouldn't see a reason to switch from 770 to N800 except you:
a) need SD support
b) experience out-of-memory condtions on your 770 now
and
c) use you 770 more at home now

The 770 does have a few advantages over the N800 that have been mentioned: hard-cover with instant off/on, much better hardware keys, and many people think it simply looks better.
The technical advantages of the N800 memory- and CPU-wise are undisputed, but might not be worth $400 except a) or b) apply. Other minor changes like the built-in stand or the camera are even seen as drawbacks by some; the camera is of little use as it is now because hardly any software supports it.
Point c) above is important because if you're currently used to take the 770 with you in your pocket, the N800 is not an option IMHO. It's too much of a living room device.

Thank you all for your thoughtful responses...

I now have a bit more to chew on.

Cheers!