With that size file image file I would go with a new Mac Pro and load it with much as you can afford. And a nice 500GB external Hard Drive will prevent you from ever loosing those great files.
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All harddrives are prone to failure. Internal or external ones fail just as easily. I recently bought three 750gb external disks to keep as a backup. Two offsite (My parents house and work) and one at home. These are in addition to my RAID5 on my workstation.
I backup the Lightroom database/images to the external disk here once a week, (images are stored on a small wd passport and a Wolverine ESP also) until i can move the backup to the other locations, about once every 2-3 weeks.