It really depends on how you use your computer.
For instance..
If it were my 20GB Drive, I would:
Format 25% (05GB) of the drive for the OS (WinXP) - (c

NTFS/Fat32*
Format 75% (15GB) of the drive for the Storage&Backup - (D

Fat32
Note on Partitions*:
NTFS is NOT accessable with a normal 9x (95, 98, 98se, ME) Sartup Disk.
It is accessable through the CD-Bootup "Repair Console" -=as long as you know the password!!=- -(If requested)
Fat32 IS accessable from a normal 9X Startup Disk and also the Repair Console. Making it easier to recover data and make 'needed' repairs. (That is why Storage&Backup data should also be this file type.
Note on space usage:
In my opinion, if windows grows larger then 5GB. I should probably re-install anyway

Also, I like to "install" programs to a drive/partition other than my boot drive/partition. Thus cutting down the ussage of C: for program installs.
But it is really your preference. Try to plan for what you think you'll need in the future.
Hope some insight.
James