3.1 is minor bugfixes over 3.0 really, plus a few extra bits, and better support for ROM patching, hence the requirement for these ROMS in OS3.5 installs.
if you set your installer to expert mode, it will ask you to confirm what it copies - it will check version numbers of stuff it will overwrite and ask for permission, but usually you're better off updating anyway.
OS3.1 will quite happily update a 3.0 boot partition, and will leave the other one alone - just dont go into HDToolbox if it asks, do that first if you need to !
you might be better off having a few more partitions if you have the space, but thats just my two penn'orth.
if you are bothered about overwriting anything, make a copy of anything you have concerns about - either put a backup drawer in C: or copy it onto another partition temporarily.
good luck, and above all, dont panic !
