I have fast internet here and with 6TB you're looking at quite a while whenever you do a cloud based backup.
I use an older version of: http://www.newegg.com/Product/Product.aspx?Item=N82E16822108123
(HDDs will be extra.)
Mine is currently populated to about 3.5TB. I use Win7 pro on most PCs. You need pro or ultimate to use the built in backup utility in windows to backup onto a network share such as the above NAS.
If all of your PCs use the home version it may be easier to buy another PC and load Windows Home Server 2011:
http://www.newegg.com/Product/Product.aspx?Item=N82E16832416443
If all or most of your data is on 1 PC the best and cheapest backup solution is:
http://www.newegg.com/Product/Product.aspx?Item=N82E16822178117
Just some thoughts. 
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