The N800 is an OMAP2 while the 770 is an OMAP1 so there are some architectural differences too (probably the most important of which is that the N800 has support for hardware floating point). In fact the standard software isn't compiled to use hardware floating point (due to the size of this code), so this doesn't make much difference unless you compile your own, and you need do do fp stuff.

The N800 is certainly quicker than the 770, however it and the N810 suffer from a slow memory controller which means that playing full-screen video is difficult - you get tearing (though I understand that some patches to Diablo may have improved that).

The most important thing is that the N800 is still supported and updates are forthcoming from Nokia. The next device is thought to be out next year, and it is far more likely that the more recent tablets (i.e. n800 and n810 which are virtually identical under the skin) will be able to run at least some of it.

So to sum it up I'd say get an N800 (actually I'd say get an N810 as I hate not having a keyboard, but there you go )