LivinFree is exactly right.. the options are endless, but just about any brand of Linux you buy today will come with a boot loader (LILO/GRUB) which will be installed with the OS... It should recognize your other OSes on the other drives and do the work for you..

Personally, I don't know why you'd want two different brands of Unix (Linux=Unix as LivinFree stated) so I'd personally just stick with a version of windows and maybe Mandrake or Red Hat or even FreeBSD.

That's just my opinons.