Onboard controllers are almost without exception 'fake' raids i.e. they don't have an own controller chip. They just provide an interface to the OS to handle raid similarly as linux does soft raid natively.
If you get a $300 raid card with it's own processor it offloads I/O functions from the processor and gives you a slight performance benefit if you use an app that has heavy I/O functions.
One of the biggest advantages to using an additional controller for your RAID array is something that's rarely mentioned - it's not performance (although MrRiplEy is spot on), it's not functionality - it's that you can move the drive array to almost any other computer that will allow you to plug in the card and keep your RAID Array intact. A failing motherboard in a system (or a simple desire to upgrade) is no where near as big of a deal.
FWIW, I don't see any reason to even bother with RAID 5 any more. Disks are so cheap, and RAID 10 performs so much better, it's just a no brainer.
But SSD - even SSD RAID - is IMO just not worth the 10x+ cost. And given that folks I trust (like Skuzzy) are stating that the current crop of SSD's are prone to catastrophic failure put's a done deal on them in my book.
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