500/500/500
If you want to increase reliability you might want to look up RAID.
There are several options (me myself I have 2 external NAS drives Mirrored). This makes copies for you without you needing to do anything and some raid options increase the performance of retrieving data (not always though). If you buy a cheap card to run you raid get 2 and moth ball one unless your pretty sure the company that makes the raid card will be in business for a while.
http://en.wikipedia.org/wiki/RAID