Once you have the space available, let the boot loader do its own partitions. It knows what it wants. It will also install what you need for dual booting. (at least that's what Anaconda did)
I tried using Partition Magic to set up my Linux partitions, but it wasn't necessary. Just give it enough unpartitioned space to do its work.
I'm currently using Mandrake ... would Anaconda be better to use?