I'm a little confused, normally onboard means built into a motherboard, which mb are you looking at. And a hdd controller either does raid or it doesnt, there is no how good does it do raid, (well linux software raid) but i assume you are using a flavor of windoze?
I'm looking into MicroStar's soon to be released K7T Turbo.
I've seen other On-Board RAID criticized as poorly implemented. Other's have since suggested the Promise would probably be a good on-board implementation due to it's apparently respected reputation. I don't know if I'll even use it, but maybe.