Just a couple of thoughts (background... I shoot a 5DII, have 750GB of images, and have a MacPro with 6.5TB of internal storage + external no-caddy drive bays for backups).
Don't buy 350/500GB drives. They're older technology, and they're no longer the most cost effective in terms of GB/$.
The sweet spot for GB/$ is in the larger disks - 1TB to 2TB - and these disks will also be faster, as the data is packed tighter on the disk surface. You'll read more bits off the platter for every disk revolution.
Make sure you have a good backup strategy in place - and design that from the very beginning. Back up to external drives and try and keep a copy offsite. I also use an internal partition as a TimeMachine disk - just for my boot drive.
Edit. Oh yes... one more thing. RAID IS NOT A BACKUP... and don't trust anyone who tells you it is. RAID will ONLY protect you against disk crashes, NOT against accidental deletion, theft, water damage, electrical glitches, virus action, fire, accidental damage which together are more likely than a disk crash. To be safe, you need three copies of data, one of which should be in a separate location.
Another HDD space junkie! I will buy 1 or 2 TB drives for the Mac Pro. However, what if I buy 1 TB drives and they puke before I fill them up? I am not being a smart @@S, just asking a legitimate question, as that scenario is my only concern. Also, thanks for clarifying that RAID is not for backup. my good friend uses RAID to backup, but he never writes to his drives, only reads. They are for storing tons of movies and games. Off-site storage is a big deal with photographers and I understand the benefits, but I never would have added 1+1. LOL
I am still thinking about my backup plan, but I have plenty of options thanks to the photographer community! As I stated in my last reply, I will clear my mind this weekend and in all likelihood, order a new Mac Pro 8 core soon.