Need help with a USB 3.0 case

yep, it will work no problem. you will get 2.0 speeds obviously cuz that is what your motherboard can do. If you can ever get a MB with 3.0 usb get it. It is MUCH faster than 2.0.

Edit: just remembered that USB 3.0 is 5gb per sec. 2.0 is around 480mb a sec. Thats 10x the speed increase

If i plug the USB 3.0 cable in the USB 2.0 slot on the motherboard, will it work like a 2.0 USB?

You cant plug an internal USB 3.0 cable into a usb 2.0 motherboard header/connection. They will not fit.

You can plug a usb 3.0 device into a 2.0 slot and vice versa.

I would imagine so. Almost certainty, it is all supposed to be backwards compatible.
If you wires that come out of the ports in the box look like they would fit on your motherboard, I would say yes, for sure.

USB 3.0 cable support the old USB 2.0 slot.

USB 2.0 and USB 3.0 internal connectors are physically different different. Yo need an adapter to plug the case USB 3.0 connector into the motherboard USB 2.0 connector.

Just buy a cheap USB 3 card and stick it in an open slot on the motherboard. Most have at least 1 header for front panel/bay mounted stuff. Just make sure the one you pick does and you should be taken care of.

On second thought, I found the full specs and better images of that board. Buy a different motherboard. Seriously. 1 PCI slot and one PCIe slot with no USB 3 on the board just isn't going to cut it. Many companies make many boards in the same price range or only a hair more expensive that are MUCH better. This looks like one of the boards Asus builds for companies pinching pennies and stamping out a bunch of workstations.

Buy a PCI USB 3.0 Card connect the front header ports to the card.