I use a 10GB size for my C:\ partition that contains XP SP2 and all my applications. I've got a good many applications installed and about 6.5GB are in use (3.5GB free). All my games are installed on the 2nd partition (D:\) and all data, downloads, pictures, MP3s, etc., are on the 3rd partition (E:\)
Unless you are planning on installing several apps that eat up the drive space, 10-15GB should be fine for just the OS and main programs.