i am not anywhere near an expert but my son is and i will try to paraphrase what he told me when i wanted to do the same thing.

My iMac does not come with Raid support and i need to buy an external drive with raid built in. he found wd20000h2q 2tb drive to have Raid built in. he set it up as raid 0. the Apple store in Tampa has it for 329 (as of a couple of months ago). i got it on the internet for 299. works well but the connector is touchy and any movement of the drive and it goes off line. not a problem, if is leave it alone.