I currently have an Asus Nvidia GTX 690 which i was think about getting another one for sli set up. I will be building my windows 8.1 gaming pc soon and would like to use the fast boot feature of windows 8 which you need a uefi bios motherboard, hard drive, uefi vbios graphics card along with windows 8. My GTX 690 does not support uefi vbios out of the box but there is a flash uefi update tool via the asus website. As i have not built my gaming pc yet i installed the 690 into my sons pc to flash it to the uefi bios but because his motherboard does not support uefi i have to buy a board that does and some other things.
My main question, rather than getting another 690 do you guys think i should go for a 2x Gainward NVIDIA GeForce GTX 780ti Phantom sli (these cards come out of the box with uefi bios) or titan Black sli or just get another 690 and flash both of the cards.
Well, 780ti sli would be optimal and I would go with that if possible. I wouldn't sli 680s because it will essentially be similar to quad sli 680 and games don't scale very well to 4 gpu or even 3 for that matter. Sli titan would be a waste as you would be paying a lot more than sli 780ti with minimal gain. Resolution is also a factor. If you are only gaming on 1080p screen, no need for anything better than 690 at the moment.
A single gtx 780ti will play everything very well at 1440p. Only crysis, AC, metro last light put a single 780ti under 60 frames. A 690 would also do very well at that resolution. I don't see much gain from adding another gpu or changing your 690 to anything right now. At most, I would not go higher than sli gtx 780 @ 1440p.