I could use an external drive as the boot/applications drive which would free up all 4 bays for HDs. Would a 3 drive RAID0 be noticeably faster than a 2 drive RAID0?
I don't think I'd want to keep scratch and data on the same drives, even if they were partitioned. Wouldn't that slow things down if they're on the same drives?
Another option I didn't think about was to get the adapter from maxupgrades.com that lets you install two more drives underneath the optical drive for a total of 6.
I use the MaxUpgrades and run 6 drives total and one optical, and I use the software raid built into OSX not a card.
I have a pair of WD 640 Black drives in the Maxupgrades slots in RAID-0 as my OS. I then have 4 WD 640 Blues in the 4 main bays in RAID-0, the first partition (fastest) is a small 30G partition of each for a 120G RAID-0 scratch volume, then the remaining 4 main large partitions in RAID-0 for very fast image storage and access. (I even left 150G at the end of each drive for use as single volume spaces for back-up copies of my OS.)
Now that we have all that settled (), yes, you will notice a speed improvement going from a two to a three drive RAID-0, and will even notice about half-again as much boost going from 3 to 4-drive RAID-0. This assumes that your files are large enough to have CS tag scratch regularly, but regardless, the performance boost is noticeable throughout the system.
You could keep data and scratch on one large volume, however over time, performance will suffer. The reason is disks basically write data from the outer faster part in to the slower part, so over time as you load data on a drive, the I/O performance is slower simply because you are writing to and reading from a slower part of the drive. If your drive is 3/4 full of images, then scratch is going to be occurring on the slowest 1/4 of your array. Granted, it's still going to be relatively fast compared to a single drive, but nowhere near as fast as if you had dedicated the outer rim area of the array to scratch.
EDit: IMO you do not need to buy enterprise class drives for this either, but all drives in RAID-0 should be the same size and model number. I happen to use WD 640 Blues in the 4-drive array, and a pair of WD640 blacks for the OS RAID-0. These drives are only about $70 each now.