well the 7990 isn't released yet! But there are some announced 7970X2 cards.
QuadFire 7970 seems cool. 8x PCi-E 3.0 is also nearly enough. But i wouldn't want to run that without WaterCooling^^ At least not on reference Cards. Maybe with some realy cool Custom design Coolers. but most of them are higher than 2 Slots. Or even with 2 Slots and 4 Cards you don't have enough space between for efficient cooling!
ONE GTx690 is always to preffer insteat of two 7970! Thats because Nvidia has a built in AntiMicroStutter device in their GTX690!
http://pics.computerbase.de/4/0/9/5/0/diagramme/104.svgz
But i have NO idea how that will work with Two 690 ! Also, with Two 690 or Four HD7970 you should be over 40FPS the whole time! (At least if your running 5760x1080 or something Similar) and not getting problems with micro stuttering.
Oh, and there will be a HD7970 GHZ Edition! that clocks with 1075mhz instead of 925! IF you go quadfire 7970. You might want to buy 6GB Edition (Hella expensive! and 3GB will nearly do!)
The Problem of the GTX690 is the 2gb Vram! It DOES limit! Not on 5760x1080 with 4xAA, but if you have a higher resolution, 8x or 16x AA You *could* hit the Vram Limit!
And maybe in 2 Years games will use way more memory ? Nobody Knows, but if you invest so much into a new PC you'd be dissapointed if you get 2-5FPS cuz of Vram Limit in 2 years!
I haven't heard of a 4GB GTX690 or 6gb HD7990. So using SingleGPU Cards might be the only way to get a future proof system!
So GTX680 VS HD7970 If you get the GHZ edition with 1075mhz or more its *nearly* als performant as the 680. but still has 1gb Vram More (2gb if you get the Doubled Vram Editions!) and IIRC the Radeon Cards run faster in Extrem High Resolution! So whats left is QuadFire/QuadSLI support! No idea which one is better. Just because a game supports Crossfire there is no guarantee it will also work in Quadfire!
The benefit of SLI is that you can use ALL GPU output! on Crossfire you have to plug ALL monitors on one Card.
and there is also the Surround vs Eyefnity discussion. (
Read AussieTimmehs Article about that)
Well, hope i gave you something to think about ;)