i have a couple multi boot systems running win2k and Gentoo linux...
ive only used RH and Gentoo heavilly enough to talk about the install process, but mandrake being so close to RH would probaly be very similar... with RH8 and 9, all I did was use the graphical installer and select the second HD as the drive to hold linux. the installer did the rest. very easy.
with gentoo, the install process is much more invovled. you have to boot from the liveCD and run fdisk to slice up the disk. Then you pick what file system(s) you want to use on your partitions. Next comes the bootstrap/compilation/portage/kernel setup of gentoo. Then you install the bootloader (grub), at this point you install the grub rec on the MBR (for my setup, the first HD was windows, thats where i put grub/MBR). Then you install packages.
of course this is not meant to be an actual how-to install gentoo, thats what the very well written gentoo docs are for! if you do go gentoo, read the docs carefully! if you do everythign by the letter (and not even skip a single paragraph) you should have a working gentoo install.
as for tweaking the bios, I think if your windows drive is the primary master, and you install the grub rec on that drive, you should be ok. I think that is, you migh twant to research that further.