At 20x30, if you use every single pixel you'll end up with 140 PPI. It's certainly within the realm of possibility, but your technique and framing doesn't leave any room for error compared to the higher MP cameras.
That said, I've seen 20x30 and even larger enlargements from a D40x, and the optics and pixel pitch of the N1 system definitely support the required sharpness, so, go for it.
I have done 30 x 40 with a 3MP Coolpix 995 and they looked great. I have sold a bunch of them even. I use a few Photoshop tricks to get the file size large enough. PM me if you want the gory details but the only way I know how to do it is with Photoshop.
Also keep in mind there are some tools out there, such as onOne's Perfect Resize that can help resize an image upwards. Of course this is no substitute for having the resolution there at the start but it is a better method than using photoshop's resize and better than spreading the pixels over a wider area.