Assuming you're actually in the developer program (so we can continue talking about this...), you'll want an external FireWire or USB harddrive. Then you use Disk Utility from your original system DVD to restore your internal HD to the external (select "delete everything" in order to get block-copying instead of file-copying, makes the copy-process sooooooooooooooooo much faster!). Then you can install Snow Leopard onto the internal drive to test it. After a while, you'll want to go back to your current system, then you simply restore the external drive to the internal, just like you did at first, but the other way 'round, and you'll be back where you began.