Format HDD

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Boot from the OSX Disc and do a format the disk using Disk Utility. Make sure you tell it to "Zero the Drive" (I can't remember exactly what it says buy it has something to the effect of setting the switches to zero). This will most likely take a few hours to complete but when it is done you should not be able to recover any information from the drive. After that, install a fresh copy of the OS (if you want to ship it with the OS installed) and you're done.

 

No, the switch from Leopard to Snow Leopard isn't a big overhaul in terms of the UI (user interface). However, it's supposed to be a big change for the general code base of OS X. It's reportedly being optimized to take advantage of the multiple processor cores in the latest generation of Intel Macs. In that regard, it's going to be a significant speed boost, but, nothing earth-shattering for the average Mac user. If you purchase the Macbook after Snow Leopard is released, then, the OS will come pre-installed at no extra charge. But, if you purchase a Macbook today, you will have to purchase Snow Leopard when it's released sometime next year and install it on your Macbook.