two 690's (wich means 4 GTX 680 GpU's) will outperform 2x r9 290x by quite a margin, but the problem is the amount of usable VRAM buffer...nvidia made the 690 with only 4gb onboard (2gb per GPU's) so it's a 2gb usable card, you put two of those in SLI (meaning you're doing 4 way SLI) and you still get only 2gb of VRAM available for use...and this at 4k resolution will not cut it...if your intention is to play at 4k resolution i would look for 2x titans black that way you get 6gb VRAM buffer usable or if budget become an issue with 2x 1000$+ GPU (may happen lol) i'd suggest 2x GTX780ti you will only have 3gb of usable VRAM but you will have enough GPU power to push the latest games at over 60fps maxed out at 4K resolution...2x r9 290x is not a bad option either, you would get 4gb of usable VRAM but the GPU power will not be sufficient to max out some games at 4k, and crossfire from experience is a lot more pain in the a$$ then SLI and suffers from microstuttering and is optimized in much less games than SLI...i would not go that way because of that...