RGB is color shown through light, like on a computer screen or on a TV, while CMYK is on paper.
You can go from RGB to CMYK without making the cover look different if you use a good ICC profile. I can't help you with that, cuz my english ain't top notch, and I have a swedish version of Photoshop, but if you find a manual or turitorials on the web it ain't so hard.
And like ikIndividuoh said, you should make it a bit bigger so they can cut it out without getting a white stirpe around the cover.
300 dpi will do.