Originally Posted by MrSingh
I can use 2x crossfire and 1x sli in the same setup??!?!
No, you can't have 2xCrossfire AND 2xSLI at the same time. What was suggested was you use the 2x6690 for all graphics, and add one GTX580 EXCLUSIVELY for Phys X. However,, I suggest you look at the list of Phys X games. There's not many out there nor are there many upcoming Phys X games. Phys X just isn't that popular. Even then, I'm not sure it would be worth the risk of software issues with that type of set up (AMD & Nvidia graphics drivers)
As for which card(s) you should use, additional information would be helpful.
(1) What's your monitor(s) resolution? 1600p & higher (maybe even 1440p) the little extra VRAM in the 6990 (2GB for each GPU vs 1.5 for the GTX580) will help along with the 6990 greater graphics power. At 1200p/1080p, the GTX580 1.5GB VRAM is enough IMO.
(2) You mention "computing power in video rendering and image editing is a big priority." Ok, what software specifically are you talking about? If your using any Adobe Creative Suite application other than Photoshop
- then it's Nvidia by default if you want GPU acceleration. However, if it's JUST Photoshop for Adobe and something else like VegasPro, the 6990 or 580 will work. NOTE: AFAIK, you will NOT get any benefit from either Adobe software or VegasPro for more than one GPU (i.e. only one GPU on a single 6990 would be used). I strongly suggest you determine whether AMD, NVIDIA or both will work with each one of the GPU compute/acceleration software applications you plan to use. Finally, IIRC, I think the GTX500 series was much better at GPU compute than the Radeon 6000 series with the roles reversing for the current generations. So even if you can use AMD for your GPU compute/acceleration software, depending upon how important max GPU compute/acceleration is to you, you still might be better off with 2xGTX580 in SLI. This question may make the choice for you.
As for the CPU, I doubt you'll see any bottle-necking with any sort of decent OC. While it can happen, IMO that fear tends to get overplayed a bit. It's not like your running an Athlon.
Lastly, nice PSU. The AX1200 is a solid choice. The AX1200i is even better, but more expensive.
Short version = IMO, unless you must use NVIDIA for GPU compute/acceleration due to software limitations or maximizing/prioritizing GPU compute performance, I'd go with dual 6990s (just make sure your case has good airflow to the cards). With that set up, you should be good for quite awhile IMO up to 5750x3600 resolution.