I'm not positive but i dont think 2MB buffer and 8MB buffer hard drives will work togethor in RAID. I may be wrong and they may work "ok" but i dont think they will.
And if you RAID0 those drives togethor you will only have 40GB total so theres no point.
Putting them in RAID 0 will give you 40GB of storage total. Also the 2MB cache drive will bring down the performance quite a lot. You'll get better performance from your single 80GB drive then from the RAID array.