My 2 cents worth:
5 GB for WixXP. (small volume to defrag)
35 GB for your Apps (again not too large for descent defragging)
40 GB for your files (don't really need to defrag)
40 GB for Ghosting (saves your stuff if you have a crash and can't recover data)
Don't forget that however you partition it you can rearrange the drive letters to your own specs so long as you do it before loading any apps or files, WindowsXP dosen't allways assign the drive letters in a logical sequence.
Hope this helps.