I don't know the whole nokia model line, but if you plan to use the N800 more for online than the actual phone, any 3G clamshell phone would work. Find one that has a compatible extended battery accessory and you should be good.

My job never has me more than 50' from a computer so tethering my N800 to my Motorola phone to get online is a bit redundant. I can say that installing Phonelink on the N800 has made replying to text messages a hell of a lot easier.