UcouldBrong is right, you will start to see deminishing returns after two cards. Even though, it's not as bad with the newer HD 7000 and R9-200 series, it's still apparent.
It looks like you are running a single GTX 660Ti with a GTX 260 for PhysX right now. You'd see a nice performance boost with even one R9-290 / 290X. Heck, even a R9-280X (aka HD 7970) would be a jump in performance.
Stick with your TX850 and grab two R9-280X...or..if you have the cash, two R9-290 / 290X.
Even though your motherboard runs X16 / X16 / X4 / X4, PCI-E X4 shouldn't be much of a performance drop. I would rather worry about if your case is large enough to support three GPUs.
If you are still "iffy" then your options for... X16/X8/x8 (or something similar) would be
- ASUS Crosshair V Formula-Z
- ASUS Sabertooth 990FX
- MSi 990FXA-GD80
- Gigabyte GA-990FXA-UD5
- Gigabyte GA-990FXA-UD7
- AsRock 990FX Extreme9
Depending on what GPU's you decide to run in three-way CrossfireX, you may either need a 1050W, or a 1200W+.
Have a look at this: https://www.youtube.com/watch?v=TISoJsWbaSc
The guys at DigitalStorm did some R9-290X 1x, 2x, 3x, 4x CrossfireX testing late last year. Pretty cool stuff IMO.
Phenom II X6 1090T @ 4.1GHz FX-8350 @ 4.8GHz // ASUS Crosshair V Formula 990FX // Sapphire Radeon HD Dual-X 7970 @ 1150/1500 // Thermaltake Frio w/ push-pull (using Antec Formula 7 Nano-Diamond + Cooler Master JetFlo fans) // 8GB (2 X 4GB) G.Skill RipJawsX 2133MHz // Corsair TX850 850W // Corsair Force GT 120GB SSD (OS) // Kingston V200+ 120GB SSD // WD Caviar Black 1TB // Windows 7 Ult x64