My setup on my main computer is:
80GB drive: 20 Linux (ext3) , 60 WinXP (NTFS)
120GB drive: 120GB FAT32 (setup before Ubuntu had NTFS support, may switch to NTFS if I can backup all the data to DVD's)
On my Compaq (Sempron):
60GB drive: 20GB Linux (ext3) , 40GB WinXP (NTFS)
On my laptop:
30GB drive: 15GB Linux (ext3) , 15GB WinXP (NTFS)