I used to own a Nikon D800E and the Sony RX100, but now both have been sold. For someone wanting to entertain the idea of 24x36" high quality prints (let alone museum quality), Fragilefinger, your deciding on the D800 vs the small camera was definitely the correct course of action in my opinion. As good as the RX100 was, at large prints sizes it's not going to compare very well to a D800 with a good lens. The challenge you might face with the D800 is getting high quality lenses for your 24x36" prints.
But I felt the need to downsize, because as great as the D800E is, it didn't give me the quality at 20x30" or larger that I wanted in return for lugging around a DSLR. Many people find the D800 (and E) is more than enough for 24x36" and even larger, without stitching even. I was used to large format film (4x5 to 8x10 color) and thinking the D800 would be close enough to that quality or to medium format digital (which I cannot afford) I took a spin. But I'm not the most expert at preparing files for digital printing and I use commercial labs - Chromira and some Epson high injets on occasion. With my eyes a few inches from the print I just don't see the quality I am looking for. I know, kind of ridiculous, as most large prints are viewed from 2 feet or more. But I'm trying to please myself more than others.
One problem I found was getting the right lenses for the D800E. High quality across the image in a focal length that I preferred, which is around 35mm, is what I was looking for - for landscapes. I found the best was the Zeiss 35mm f/2, with the Zeiss 1.4 better at f/8 and f/11 a little bit, but worse otherwise and more variable from sample to sample. In any case the latter lens was not worth the money to me, for my landscape purposes. Also when doing non landscape shots I found the manual focus to be a pain. Having a tripod and doing live view for everything reminded me too much of my large format days - although admittedly far far easier overall, but then I've become ever increasingly lazy, and I just want a camera I can take pretty much everywhere. I tried the Nikon autofocus lenses them wanting. For example the 35mm f/1.4G was not sharp outside the central area until stopping down to f11 (this is with pixel peeping at 100%) and the lack of sharpness was asymmetric, so maybe that sample was particularly bad.
So, now I'm going to have some fun with the Sigma DP2M (a not easy to use camera, and even more work in post processing). The results are probably going to be inferior to the D800E, and 45mm equivalent is not my favorite focal length, but I want to enjoy something different and lighter for awhile. Digital cameras are improving, and I'm sure there will someday be an affordable digital camera that will give the results I'm looking for and which will make lugging around a DSLR worthwhile for me.
Bottom line though, the D800 is going to give significantly increased image quality over the D300, even with a non-optimal lens, so as I started out with, getting the D800 is a good choice.