Usb 3.0 motherboard usb 2.0 case

They are compatible. Just use adapter like this. (if case do not have one on)
http://www.ple.com.au/ViewItem.aspx?InventoryItemId=608660

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Most boards still have 10pin[2x5](8 actually used for 2 ports 1 is a place holder and a missing one to prevent reverse plugging.) usb 2.0 plugs anyway.

As long as your board has that you will not even need an adapter.

If you had a case with only usb 3.0 the adapter above would allow the 2.0 ports to use the 3.0 plugs on the case.