I have to disagree, the basic rule of thumb isVanderJ, that is not correct as to the designing in CMYK. You will have a larger printable gamut when designing in RGB. When you design in CMYK, you are losing colors that our printers will reach. Design in RGB and allow the rip to bring in the colors to what is obtainable. I understand why logically people think that, but with a correctly profiled machine and a decent rip, you will obtain much a much greater printable color palette in RGB
Print CMYK
Web RGB
You can have your screen, proofed, equipment all calibrated to match each other via CMYK....Ever hear of G7?