1. The 450 will be marginally faster than the 300 or 150, but since you only will be using around 50 GB on the disk, it seems like a waste of money to go for the bigger one.And yes, use this Velociraptor for OS & programs. Remember to dectivate and uninstall CS5 before you install it on the new drive.
2. If you need a controller, you might as well get a raid controller. You are not obliged to define raid arrays to use the ports on that controller. On-board raid 0 is about as fast as from a dedicated controller, but raid5 is much slower from on-board.
3. New Velociraptor as boot. See #1.
Disk 1 and 4 can only be configured as JBOD, which will not give any performance increase, I advise against it. Continue using them as separate disks.
In the future, if you add additional disks, you can go for raid configurations, but for the moment I would leave them as separate disks.
I really appreciate the input. If I may, I have one last question about which controller to get: Either a Highpoint RocketRaid 2300 or Adaptec 2241000-R?
Here is a link with both shown on the same screen.
Neither IMO. Only the Areca ARC-1210 on that same page. Both the Highpoint and the Adaptec are software controllers and put a heavy burden on your CPU and deliver lousy performance.
If I get the Adaptec, I will connect it to my "seldom used" drive or my backup drive. If I spring for the ARC-1210 (at over 3 times the cost), should I connect the Velociraptor boot drive to it because it is much faster? In fact, if the ARC-1210 makes that much difference, I may use all 4 connections on it.
As long as you use your disks as separate drives and not as a raid, you will not see any performance difference between on-board SATA or a connection on the Areca. The difference will be very noticeable when you use raids, then the Areca is the fastest option.