If your menu.lst for GRUB is on the Suse partition and you remove Suse, yes, you won't be able to boot directly anymore.
However, if you install a new Linux, it will automatically find all bootable partitions and add them to its own GRUB.
At least that's how it worked for me when I replaced OpenSUSE with Debian on the same HDD.
That's also how I once got a quadri-boot: just install one distro after the other with its own GRUB... ^^
The only OS that's not so friendly is windows which always needs to be installed first...
I can't say for sure that it's the same if you install it on another HDD since I never had a PC with more than one HDD.
However, I am certain that there's a way of doing it. Google around for MBR, GRUB and multiboot or wait for more experienced users.