Yes, and the block for the 6990 is pricy to say the least.
Do you need a 6990? As noted above, a single 6970 would be quieter and still provide multi monitor support.
Also, if you are going for a single 990x and a single GPU, the Rampage III extreme is overkill. It's a top end overclocking motherboard designed with 3 (or 4) gpu arrays in mind.
You can grab any decent $200 range board and it will be plenty.