Hard drive configuration

RAID 0 will effectively multiply your speed by a factor equal to the number of drives you put in the array (2 disks = twice the speed, 3 = three times the speed), but be aware that if there is a problem with any disk in the array, you will lose all of the data on the array, so have a good backup strategy.

Nonetheless, RAID 0 will definitely help your speed, and you don't need to buy a RAID card to set one up, as OSX supports RAID 0, 1 and 10 in software through Disk Utility. The difference in CPU load generated by a software RAID 0 is miniscule.

 

SSD for boot in optical bay plus 4 drives in raid0, No more play back problems here. I had a 3 HDD raid but it would occasionally give me problems while working on multi layered compositing.

don't for get to keep clones of your drives now

 

People buy RAID cards to use RAID levels not supported by OSX, especially RAID 5, but honestly you can get away with software RAID on 0, 1, and 10, because they are not complicated structures (RAID 10 is just two RAID 1 pairs put in RAID 0 with one another). You will always have a slight performance advantage with hardware RAID, but you also fill a PCIe slot, spend a whack of money, and have to deal with third-party drivers and support. For something as simple as RAID 0, software is easily enough.

 

You may want to take a good look at your memory usage, as that's faster than scratch any day (if you've sufficient memory, there's far less need for scratch).

For a scratch location, the best thing you can do is run a small SSD (i.e. the 40GB unit from OWC that goes for ~$100). You can do this for the OS as well, but this won't benefit you as much as using one for scratch (not even close - it will read applications faster, and boot a bit quicker).

For data, if you need some redundancy, you can use a level 10 array (performance would be that of a 2x stripe set, but can take 2 disk failures before the data is lost).

Fitting this internally isn't difficult either. Place the SSD in the empty optical drive, and connect it to one of the ODD_SATA ports (boot's OS X in a 2008, but not Windows or Linux). Get a Molex to SATA power cable and a standard SATA data cable for the SSD. Put the 4x HDD's in the HDD trays, and then use Disk Utility to create the array (can make a 4x disk stripe set or 4x disk level 10).

RAID or not however, you still need a proper backup system in place (go external).

A good UPS is advisable as well (Line Interactive unit with a pure sine wave inverter - refurbished works just as well as new, but saves a considerable amount of money).

For a stripe set, you can get a bit faster out of a RAID card due to the cache (some are noticable, but it's due to the fact those cards use a DIMM slot, and can go to 4GB of cache). It's not cheap however, and isn't worth it for a stripe set, unless you're using more disks than the MP can handle or SSD's (exceed the bandwidth available on the ICH).

So the primary reasons to get a RAID card is for levels OS X can't do, or more ports. Features too, if you really understand the difference between software and hardware based RAID systems (features such as online expansion, modification of the level without data loss, and improved recovery options).