Faster hard drives mean the access time is quicker, so yes, 7200rpm is about the minimum speed for multi-track audio recording. Not sure about using separate drives for samples and project files - it all depends on how well organized the drive is. If you've got enough RAM then you should be able to get away with one drive for everything, as the samples and audio will be cached in the RAM when the project opens, so in theory you're not accessing the HD as often as you think.
Record to just one drive at a time (don't use external FW drive AND internal drive when recording tracks) or most systems tend to fall over.
As far as backups go, as long as you do it regularly then that can be whatever speed you like, as it should be done as a dedicated housekeeping task AFTER you've closed Logic, not at the same time.
I haven't used Logic for a long, long time, but you should be able to transfer projects to there new drive by using [i]File>Save a copy as[/i] and then sending that to the new drive. Make sure that if you're asked, you save copies of all the audio takes to the new drive too.
Word of warning though, when you're about to start using a new piece of hardware it's always worth doing a complete backup!
You could make your life easier and more organized) by removing any audio unused regions as part of your backup process, you might be surprised how much space you'll save.
Sorry it's a bit long winded, but I hope it helps.