I have read that you have to wait at least 3 months after uve used henna if you want to dye your hair with chemical hair dye. Don't take my word for it tho, I have never tried it, but I have a friend who did. Her hair turned out fine. Although it might have been because she dyed her hair black and her hair was really dark in the first place, so the henna only gave a red sheen to the hair.
There are some horrible stories on the internet about hair turning green after applying chemical hair dye on to hennad hair, so I wouldn't try it out on my head at first I guess =P maybe you can do a test round on hair thats been saved up in your hair brush or whatever if you do decide to use henna and you want to change it up later