I have seen no benefit with this. I prefer partitioning based on media. So Programs and OS on one partition, movies on another, and mp3s on a third. Then if you do things like digital video etc. In the end it's all about orginization.
You may see a small performance boost if you install your programs to another hard drive (not partition) than the operating system however partitioning is basically for your orginization IMO.
But... if you would like to do this, I'd say give windows 5 gb (10 if you are feeling generous
) that should be plenty. The rest can go to your applications.