If I were you: 40GB for 7, 30GB for XP, 100GB for audio-video and misc stuff, and put what's left for games..That's me, you can do it the way you want and it does not affect the health of your HDD, ok?
Get a second hd for Win 7 - This way if one of the drive fail, you still have a working os.
You also have a couple of options on dual booting.
1. You can have both hd's connected when installing your os's. This will give you the option of booting (Boot Menu) into either one at the same time.
2. Unplug the second drive - install your first os. Now connect the second drive & unplug the first drive - install your second os. When completed, plug in boot drive's.
You'll have to use systems boot menu to chose between either the first or second hd to boot from. This keeps the second os hidden from others. By default your system will chose the hd you set a your first boot device.
The second option is best if you have other people who use you computer and you want to keep them out of a particular os. My wife uses my pc and when she turns it on, all she see is win xp.