Storage and Backup Options for iMac

I just received my iMac 27 inch quad core, 1TB internal. My next step is to address storage backup options.

I am looking at at least three options:
1. 2TB external drive, 7200 RPM
2. New Technology Guardian Maximus RAID 1, 2 x 2TB 7200 drives
3. Drobo 4 bay system (not sure about this since it is a pricier option)

The IT person at my workplace mentioned that the iMac can only do a RAID system like Drobo. Did not say anything about Guardian Maximus. Does it mean the Guardian will not work? He also said that its a waste of money to buy a RAID since Firewire 400, 800 and USB cannot utilize any drive over 5400 RPM. I am assuming he was referring to read/write speeds, no? Using FireWire 800 connection, what is the speed difference between a unit that uses a 5400 versus 7200 RPM?

I also will be editing Photoshop images off of whatever external option I end up getting. With regard to using Photoshop in this manner, is there a performance difference between a single 2TB external versus a RAID?

well all of them would work the RAID doesn't really matter to your computer because it is not your computer managing your RAID it is the controller in the tower that is. usb has a limit of about 25MB/s ish, so like the 7200rpm drives would be a bit overkill. you could go with something more like 5900 or 5400 3.5" drives. they are cheaper anyway.