I don't think that there is a problem with using a larger than 120 GB drive on your QS.
On my DA, however, there may be, without the assistance of a HD controller, such as the ACard.
Also, while you can use a large drive in your Mac, you will get better performance if you buy another HDD controller card, such as a SATA or a ACard IDE controller. These are higher performance cards, and are reasonably cheap. ( go to macsales.com and you will see what I mean. )
Max.