What about 2* 2TB drives in RAID 0 for speed and 2* 2TB internal for back-up (one for raws and one for .psd.), and for the ultimate, 2* half GB SSDs in RAID 0?
Here is another thought -- Hitachi just released some 3TB drives that are VERY fast and cost about $250 each from OWC. My next box might be a pair of SSDs for OS and Apps, then 4 of these spinners in the main bays partitioned thusly using the built-in Mac software RAID:
1) small top partition off all 4 in RAID-0 for scratch, say 64G off each for a total scratch volume of 256G.
2) Larger partition on each, now 2.8TB used in a 2>2 RAID-0/1, so 5.6TB of really fast data storage, mirrored to an identical array, all internal.
Other option is three of these 3TB drives all in RAID-0 partitioned with a small scratch space on the outer rim, then the huge remaining image array all backed up to my existing external, leaving an open SATA port for the Blue-Ray. But no biggie since there are some relatively inexpensive internal PCIe SATA port cards that are bootable.