The factory OC'd 560 Ti's perform very close to the 570's. Guru3D uses the following games in their 1920 x 1200 test suite, COD-MW, Bad Company 2, Dirt 2, Far Cry 2, Metro 2033, Dawn of Discovery, Crysis Warhead. Total fps (summing fps in each game) for the various options (single card / SL or CF / Triple SLI) are tabulated below
As you can see adding that 2nd 590 gets you a measly 101 fps ... perhaps this changes at 5760 x 1080
Looking again at the table, ya see that ya can get two 900MHz 560's for $430 and get 862 fps .... one 590 at $700 for an extra $270 gets you a whopping 19 additional fps. And as yu can see the 590 / 6990 scale extremely poorly. If ya want 590 level performance, you see that twin 560 Ti's does it much cheaper and saves $270. If you want the best performance, based upon the above, you are talking 2 or 3 580's. Moving to 3 cards, the Nvidia cards scale better and are less problematic in 3 way than ATI does.
With just 14% additional fps scaling, Metro 20233 performance would be expected to be about 25 fps # 5760 x 1080 .... However 3 monitors at 120 MHz, now that's a review I haven't read but I'd expect 12-15 or so fps.
MoBo - The board has 3 hi bandwidth PCI-E slots. That board is best for twin cards in SLI (560/570/580's) plus a 3rd card for dedicated PhysX usage. It says x16 x16 x8 but doesn't list x8 x8 x8 so I cant figure how SLI works w/ 3 cards with one at a different bandwidth than the other 2
RAM - Again, most expensive will not necessarily improve your performance. As you can see here, > 8GB doesn't help gaming.