Chris,
I've tested multiple configurations of SSD's and spinners for scratch and OS/Apps on my 8-core 3.2. Here is what I've found:
1) RAID-0 SSD's for OS and apps makes virtually zero difference in overall performance over a single (good) SSD. I'm using the OWC RAID SSD's. No improvement in boot speed that could be measured, no improvement in program launch speeds that could be measured, and no significant differences in overall system responsiveness that could be perceived. In short, if there were gains, they were not significant enough to justify the expense of a pair of RE SSD's on OS.
2) For scratch, I found that a pair of SSD's performed about equally to my 4x spinner RAID-0 scratch partition when I force fed it a huge file to really stress scratch. Now with lots of RAM, even though CS5 wants a scratch partition, it doesn't need to I/O actual data to it very often except for large, multilayered files. So again, at the end of the day I didn't feel there was any gain and wasn't worth it expense wise especially now that CS can utilize RAM more effectively -- even though it sounded cool.
So here is my reco: One SSD for OS and Apps -- I mount this in the lower optical bay and use the free SATA port for it. Then 4x2TB spinners in teh main bays, all in a partitioned RAID-0; the fastest outer rim 128G of each partitioned off for a 500G scratch drive, the rest as a single (HUGE) 7.5TB partition for really fast image I/O, then those images obviously backed up to whatever external array you prefer. IMO, this will give you optimal performance and maximum storage all at the lowest cost. {FWIW, I actually partition off a third 250G inner (slowest) partition on each drive and leave them single (no RAID) for back-up data like a couple redundant bootable copies of the OS. One of these updated daily the other weekly using CCC. Having these has saved my bacon more than once recovering from bad or corrupt App updates.}
PS: Note that WD 2TB non-RE drives don't RAID well, so if you go WD drives make sure you get RE versions. Also note that the newest Seagate 2TB's are SATA-3 and offer a tiny performance gain over SATA-2 drives at a minimal cost addition. (The 2TB SATA-3 Seagates are on sale today at my local Fry's for $179 each.)
