Well why x86 particularly?
To survive in the desktop market an eventual shift to x86 is pretty much inevitable.
Rather, if it goes onto something like this, it needs to be StrongARM.
I'd go with Intel's implementation of the StrongARM - the XScale. 400MHgz - 1GHz last time I heard.
The 206MHz StrongARM in my iPAQ is pretty damned mean. There are 3d games to put low-end Amiga's to shame on this thing - even Quake's been ported to it. The thought of a 1GHz XScale chip at the heart of a decent tablet is extremely tempting. I'll go with what you say on the keyboard... I'd accept such a machine so long as I can add a keyboard via USB.
I'd want a minimum of 800 x 600. 1280 x 1024 pro'lly asking too much. At the size TabletPCs are I'd be looking for it to take over some of the tasks of my main machine - for that I'd want a decent resolution.
I've often wondered about an XScale based machine and what could be achieved if Amiga and RiscOS Ltd. got together for mutual gain. At least there seem to be a reasonable range of companies designing and selling complete StrongARM based sollutions for the RiscOS market.
They wouldn't have to co-operate OS wise... but from a hardware developers viewpoint the idea of being able to sell the same machine to both AmigaOS and RiscOS users might be quite tempting.
But then, perhaps I'm thinking a load of bull ;-)