I was on RAID 1 back when I was on the Asus Rampage Extreme X48 platform and I've only seen about less than a GB being used by Intel's RST/Matrix Storage Manager. Since I was on an array with smaller platter drives(500GB) and that you need identical drives for the aforementioned array, I ditched it for JBOD and larger drives.
Now, I'd agree with marko55's point about ram being used as cache. Which also brings me to the question, what sort of RAID are you looking at and what sort of data volume are we looking at in terms of transference? If we're talking about a NAS, you're going to need a GB of ram per TB of storage. I would need to dig that info up for you, if a NAS is what you aim to build off of an onboard RAID controller.
In all honesty, the onboard/chipset based RAID found on mainstream grade platform's aren't meant for professional duties otherwise that would, in essence, mean add-on RAID cards used in data servers are overpriced. Bleeding edge technology watered down = reduced costs and thus reduced performance.
You're welcome though just trying to help out.