So does the N800. The difference is that N810 uses a micro USB connector instead of mini, and the method for activating usb on-the-go (OTG) is slightly different if I'm not mistaken. But otherwise they're similar.
No. Both have 128MB RAM. The N800 has 2 full-size SD slots, while the N810 has a single mini-SD slot, and instead of the "internal" SD slot that the N800 has the N810 has a fixed-size, non-replacable 2GB flash where the "internal" slot used to be. That's the best way to view the situation, the way I see it.