it's simple...
Yes, have OWC add an eSata port. It only costs about $100. It is staggeringly stupid that the new iMac does not have one - it is really beyond belief.
Next you want a "RAID0" external drive box. That box will have TWO drives in it (or possibly FOUR in some versions). The box itself entirely takes care of the RAID. In fact your Mac WILL NOT EVEN KNOW it is a RAID drive. It just looks like one normal drive.
Makes sense?
There is absolutely no way to have RAID "using the internal drive" or anything like that. It's a meaningless idea. RAID is a box, with a complicated RAID card, and also as an additional matter a couple of cheap hard drives. RAID has no relationship to the drive in your Mac or anything else in your Mac.
Hope it helps!
Furthermore, with your RAID box, you will be able to choose either RAID0 (speed) or RAID1 (security). (There is a switch on the RAID box to make the change.) Again that has utterly nothing to do with your Mac. Your Mac will not even know and have no way of knowing which mode the RAID box is running under.
In answer to your other question you only need one connection (eg, the eSATA connection) to connect a RAID box. A RAID box is "one drive" overall, it only uses one cable. There is no way to use two cables, it is conceptually meaningless. Remember, a RAID drive is just "one drive" (even if there are two or maybe four physical raw drives inside the RAID box - that is something you never know about or deal with).
In fact ... every RAID box made has both eSATA and FW800 connections. So, GO AHEAD and buy the RAID box of your dreams, and plug it in with a FW800 cable.
Once you get used to it and like it, have OWC add a proper eSATA hole to your iMac. You can then change the cable from the FW800 cable to the eSATA cable.
TIP: do not try to skimp on buying a RAID box. Get the one that is a bit more than you want to pay!
