I think I can contribute usefully here, as I use linux, windows, and raid.
First off, if you use the onboard, you cannot run a raid and a normal OS drive at once. I bought a raptor and 4 storage drives planning on raid 10 with the raptor alongside and it just cant be done on a single controller. If you get a raid card, thats a different story. OS drive on the motherboard and storage on the raid card will outperform everything on one controller, especially as the onboad simply wont run both and I'm not convinced the card can either. This is based on my intel ich9, ich10r and on a long time on google.
Next, why raid 10? If running the OS from it I would understand, but with the OS on a separete drive raid 5 starts to look a lot more appealing. I appreciate the comments that the parity calculations take time and will benefit from a good card. However, if using the onboard system, it offloads parity calculations to the main processor anyway. This performance hit is difficult to measure, and definitely doesn't justify a 400 quid card.
Data moves to and from raid 5 more slowly than raid 10, and the redundancy suffers. However the capacity is far better, as soon as you have four or more disks it rapidly outshines the raid 10 economically. Further, adding more disks in the future to a raid 5 is frequently achieved while I've not yet heard of it working with raid 10.
I do not believe a raid 10 will actually show better performance on your system for most uses, if you're putting swap files and so forth on it then perhaps it is more justified. Your call, I went with raid 5.
I feel I should summarise that you'll need a raid card if you want an os drive that is not in the array, but it can be far cheaper than 400 and itll behave fine. The battery backup you mention leads me to wonder if this is an enterprise solution, but the raptor rules that out I think. A separete card should also let you move the array to a new motherboard when the time comes.
Finally, after writing what feels like far too much, how important is it that windows can access the array? The linux software raid (mdadm) is exceptionally good and is definitely cross-distribution. I assume osx can do this too since it looks for all the world like a custom unix. In particular, when things go wrong troubleshooting within linux beats the hell out of within the hardware raid config screen. Windows cannot normally see the array however.
I cope with this by running windows in virtualbox most of the time, where it can quite happily read and write to the software raid. On the rare occasions when I need windows to be able to access the hardware directly, there's enough space on the raptor to copy whatever I want from the raid anyway.
With luck the above makes sense. It would help if we knew exactly what you're doing with your system, since a lot of the above wont apply for other uses than standard desktop. Do get back to us
p.s. ubuntu read my onboard hardware raid out of the box without issue. I think the issues arise when you want an operating system installed onto the raid
