Games are data, they don't require special thinking, I have documents, games, movies, anime manga etc. on a data partition.
a huge FAT32 partition might not be very safe, and annoying (on such a huge hdd you might end up storing > 4GB files afterall)
Look into file system options : an ext2 partition should be accessible from mac OS, and from windows after install an ext2 driver. Ntfs-3g might work under mac OS and thus allow full read-write access.
I've just heard of MacDrive which looks to be a HFS+ driver for windows, claims Time Machine compatibility in its current RC; but it's commercial software that has to be bought
http://www.mediafour.com/products/macdrive8/
The internal drive could be set your way. Or why not a 20GB or 30GB mac OS partition, a 30GB win7 partition, and a data partition (ext2, ntfs or hfs+) for the rest.
Time Machine is not required for backups, you could have an OS agnostic solution like rsync, Windows also have something similar to Time Machine, called Volume Shadow Copy ; you also can do images of OS partitions.
So we have to decide which way is the simplest
. I don't mean to be confusing but I am.
External drive I don't know. Either you do Time Machine (requires hfs+) and storage, or just a big pool of storage (separated maybe as 100GB + 900GB or whatever so you keep the option of destroying that 100GB partition, making it fat32 would you need to (compatible with consumer electronics), cut it further in pieces and install an OS or whatever - knoppix partition, etc.