Personally I would not use RAID 5 in my HTPC. While I agree that protecting data is important, RAID 5 only supports 1 hard drive failure. If a 2nd drive were to fail, you will lose all your data since parity is stored across all the drives. Also by using hardware RAID, you are commiting yourself to the RAID controller on your ProBOX. Keep in mind that with 3TB drives, your rebuild times will be very long.
I would use a sofware based snapshot RAID\\backup solution like flexraid (commercial), or snapraid (free). These applications write parity on 1 physical drive and allow you to rebuild in case of failure. They are great for data that doesn't change often. Their advantage is that if a 2nd drive fails, the data on your other drives remains intact. They also support RAID6 which allows you to have 2 parity drives. If you go this route, save some money and get the probox instead of the proraid.
Here is a page comparing different solutions. These solutions have also been discussed in this forum if you searchhttp://snapraid.sourceforge.net/compare.html
If you don't need 8X3TB of storage right away, purchase the amount of drives you need ex 3 or 4 and purchase larger capacity drives in the future when they exisit and expand your storage. (note this will only be possilbe with software based RAID and not hardware based)
As for which harddrive to use, I have had good experience with Western Digital Green drives in my HTPC (non hardware RAID5 config). Though hard drive prices are very high at the moment due to flooding, I would wait 2 -3 months if possible before purchasing.