I do this..
50gb for Windows and apps (the windows folder grows a lot with windows updates in Vista and 7 to keep app compatability)
the remaining for other files and games etc.
With the 'Pictures' 'Videos' 'Music' 'Downloads' and even 'Desktop' folders in your user folder
Right click them, then place them on the second partition
I have mine like this
D:\Music
D:\Downloads
D:\Videos
etc
That way it keeps C: clean of personal files
In XP, you can get away with 10 or 15gb, but Vista/7 grow a lot, I used to say 30gb, but 50 is a safer bet