I can only speak from where I work (Among under things I'm plotting posters on a big 50" designjet at work), but I'd recommend never to seperate it to CMYK yourself.
Just leave it on RGB. The ripping software has changed and gives usally much better results when starting with RGB. Why? First, for a perfect color matching when converting to CMYK you need to know details of the used paper and used ink - to make it right every combination needs a different filter.
We use a color rip that has specific filters for exactly the paper we plan to use, and of course for our plotter. Second, if you got CMYK and you use a 6-color-plotter, there is still to calculate the 2 left colors - so additional ripping is needed anyway.
Might differ from shop to shop, but we like .tif most - or eps with fonts in it if you use fonts in your artwork, which is not that likely to appear.
I even can't use photoshop-files directly, but convert them to .tif or .eps before plotting them.
If you are not sure if your color would come right or to be on the safe side, it might be a good idea to ask if they could print you a minimized version before, so you don't get shocked when you pay $120. This way you could at least see if colors come the way you want or can happily live with.
More then 300dpi would be overkill, in most cases 200dpi is enough. As we want the best possible, I still would stick with 300dpi. Most plotters got 600dpi, some 720dpi, but as they use patterns to mix colors you won't have much benefit when you go that high.
By the way, very cool artwork, 1up!
