Mandrake's automatic installation should recognize that there is another OS on the drive and incorporate it into the bootloader. If it doesn't it's a simple matter to edit the bootloader's confg file to allow dual booting no problem.
Grub handles that very well.
I'm not sure why you want to install another distro so soon, and it's really none of my business, but I'd recommend spending some time with the distro you have before jumping into another one. Learn the ins and outs of your distro. Get comfortable with it to the point you know how to correct problems and perform all of the tasks you need your system to perform.
Then you may want to explore other distros. Just my 2cents. probably worth about half that.