Any I7-920 MOBO which supports USB 3.0 & SATA 6Gb/s as future proof?
You can always buy a usb3.0 add-in card. But it is probably cheaper to get a mobo with that capability up front. Those add-in cards are a bit expensive today. I see USB 3.0 as a very good way to connect to external backup drives.
The value of SATA 6gb is more iffy. It will be good for future SSD's. A SSD capable of using that speed is probably some time off, at least something affordable.
They will be pricey, but will need a 6gb sata to run at full speed.
If I were to buy a new motherboard today, it would include 6gb sata and usb 3.0.
As to raid-0 for the OS, don't bother. It will work, but there will, on balance be no performance impact.
Raid-0 distributes alternating stripes of data on each drive. The OS mostly does random reads and writes which will be satisfied by one drive or the other. Only if you were doing sequential operations of large files is raid-0 of much help. Then, the reads could be done simultaneously.
If you want the best performing OS, get a good SSD with trim support. A 40gb drive goes for $100-$150. Pair that with a 1tb storage drive and you will have a good system for today.