Converting to CMYK color space doesn't do you any good unless you have a color calibrated monitor that is using the same ICC color profiles that the printer has set up for their work flow, screen frequencies and dot gain for the appropriate paper and printing press that will be used to reproduce the image. You are better off to let the printer do the conversion to CMYK and ask for a digital color Iris proof. Review the proof at the printers light box (or 5000K light source, I.E. sunlight) to make any comments regarding color corrections at that time. Ask for general prices for corrections BEFORE you submit your work for reproduction! Some printers have great printing prices but make up the difference with Customer Alterations. In general, you will want to increase the levels and sharpen the image a bit. The problematic RGB to CMYK colors are mostly greens & oranges.