You should just use the one hard drive. There will not be any difference in performance. In fact, if you put windows on one drive and other programs on another, you could a little performance. Hard drives are supposed to have 10-15% free space and you'll probably end up having less space. I know the math says you'll have the same amount of space, but I doubt you'll be able to efficiently split files. And windows doesn't need 120gb. A 64gb drive would be more than enough. Also, if you haven't opened something that's installed on your larger, non-OS hard drive in a while, depending on the hard drive, it'll spin down and you'll have to wait for it to spin up again, causing things to "freeze" for a few seconds while the drive is spinning...