I am both PC and Mac user.
I partition my harddisk in my PC like this.
Drive C : 50GB -> Window XP & Programs only
Another harddisk drives -> For saving data
The reason that I did that with my PC because to protect all of my data from situation that when there is something wrong with window.

And now I'm using Mac as well. Macbook Pro, Leopard 10.5.6.
And I partition harddisk and installed OS Leopard the same way I did with my PC

First Drive 50GB : OS X and Application only
Another Drive : All data

I want to ask eveyone what do you thing about this?.