In the end I went with the most expensive but also the fastest option: #1 which was the Mercury Accelsior PCIe based SSD hardware RAID array (RAID 0 by default which I will keep) in a Mercury Helios Thunderbolt PCIe chassis. The main reason was because it was the only one that mentioned it was bootable (and made a big deal out of that fact).
I paid $799 for the 240 GB model (it's 128 GB X 2 minus the unusable amount...a nice touch for the seller not to count that). It's got a lot of technology built into the card including processors and cache etc. to monitor and control the hardware RAID array including extra error checking and load leveling/balancing. Also it's theoretically upgradable (each drive is called a "blade" and while they don't yet sell independent blades they may in the future). Also the PCIe chassis is an extra bit of the investment that could live beyond the SSD, just like the whole arrangement could live beyond the use of my iMac itself.
This is a great review on it where they say it's "really fast. Ridiculously fast."
A guy on youtube has a (somewhat boring) video of his Mac Pro taking 3 minutes to boot and launch paint shop pro with his hard drive and then after setting up one of these it cut down to 15 seconds! Crazy balls out fast.
This is the sellers sites for;
Of note is that if they didn't offer the bundle discount and if it were not so much more expensive I might have opted for this much more attractive chassis.
Or for even more and larger chassis with multiple slots and supporting larger sized cards.
These chassis got me thinking about the possibilities with thunderbolt. Now any iMac or MacBook can be as capable and expandable as a Mac Pro...there are even developments with video cards in one of these things! I do wonder if it would be possible to take a MacBook Air and hook up a thunderbolt PCIe chassis with a high end graphics card with the display on the internal monitor?
If I was one of these manufacturers, or even Apple, I would consider making a full on chassis with PCIe slots and storage areas, etc. I really wanted a Mac Pro but they were so old...now I can get some of that goodness piecemeal, though for a price. :-)
I get the device about a week from now and will post back my impressions if anyone is interested.
Best Answer
In the Mid 2011 iMacs (both 21.5" and 27") Apple left enough room for an extra SSD. So if you want to put an SSD inside your iMac you can do that without replacing the DVD drive and/or original hdd. If you're going for an external one you should indeed opt for a SSD with a Thunderbolt connection, otherwise you kinda loose the speed advantage of a SSD.
If you want to separate the OS and application data from your media files, that's entirely possible and there's more than one way to achieve that. Read this and decide for yourself wether it's easy:
SSDs will only get better and cheaper over time so it can make sense to wait, but since they'll keep getting better and cheaper you could also wait for the rest of your life but SSDs are becoming mainstream now so it is safe to make the jump.
Remember that if you install a SSD in your iMac yourself you will loose your warranty so it may be worth to let an Apple Authorized Service Provider do the upgrade for you. If you insist on doing it yourself you might find the following link useful:
Good luck!