if your board has 1 16x slot and the other 16x slot runs at 4x then avoid crossfire, it will work but the results will be disappointing, stuttering, fps drops etc. bot for 2 cards you need dual crossfire support, not quad.
I have a Gigabyte GA-MA790FX-DS5 motherboard. It supports dual crossfire. i currently have 2 radeon 4870x2 installed, things seem to run just fine, but i am wondering if i am capping my PCI-e lanes because the motherboard does not have quad crossfire support?
I wouldn't recommend you buy two 690's.. your bottlenecking your system.. if you have a AMD board.. i dont care what CPU you got from AMD.. its a bottleneck