4GB swapfile? *faints*

For Linux, I don't know what performance you gain or lose from having partitions on different physical drives. I would guess that keeping things on the same phyiscal drive would tend to be faster, but I have no idea.

So far as partitions, this is what I'd do:
- 100MB boot (ext2)
- A goodly sized root partition (i.e. /), reiserfs or ext3 or whatever (I like resierfs). For some perspective, I have 21GB /, and I really have trouble filling it. Darn Linux and its lack of bloat. There's a real limit on how much you're going to need to put in /, depending on what choices you make of course.
- An even bigger /home, reiserfs or ext3. /home is where you'll store most of your crap (movies, music, documents), so if you're anything like me, it'll take up more room than anything else. You want /home separate, because it's what you really don't want to lose in case a partition dies.
- If you care at all about security, make your /tmp a separate partition too, because it tends to be world-writable, and a rogue program can fill up your HD if /tmp isn't its own partition.
- Swap partition, size depends on how much RAM you have. If you have less than 512MB or so, then 2x RAM size is fine. Unless you're running something berserk, you aren't going to be touching your swapfile much. I have 512MB RAM and 512 MB swap, and on the rare occassion I use swap, it's usually less than 50MB, and only lasts for a second or two. Darn Linux and its lack of bloat. Swap space doesn't hurt anything though (that I know of), and its good for emergencies.

So far as backup partitions, dunno. I just use CDs, and keep a ~/backups folder.

I agree that if you want to share files between OSes, you need a FAT32 partition. In fact, I'd say install Windows itself on FAT32. That way one day when you realize you're wasting all that HD space on Windows partitions, you can easily resize them and give up the space to Linux.

Pay attention to what order you put the partitions on the disk too. Think which one you might be most likely to want to grow, and which most likely to shrink, and put them beside each other on the disk.