Which Nvidia card is the best upgrade?

[rreluzco]

I'm a fairly heavy gamer and I'm thinking of upgrading my video cards.
I'm looking forward to games like Borderlands2, Crysis3, Black Ops2, MOH War hog, BF4, Far Cry3, Metro Last Light
Looking to spend around $400-$600 on only cards. I'm hoping that my system is beefy enough that I can get away with a really heavy upgrade using video cards only

My machine totally rocks out BF3, Crysis2 (with DX11 and DX11 Texturepack), Metro 2033 on max settings, no problem. Also runs Batman Arkham city on highest settings (not as dreamy as BF3 and Metro but very very good nonetheless)
Here's my specs:
AMD Phenom II X6 1090T Black Edition Thuban 3.2GHz
ASUS M4N98TD EVO AM3 NVIDIA nForce 980a SLI ATX AMD
2ea EVGA 2GB GTX 560TI running in SLI
G.SKILL Ripjaws X Series 16GB (4 x 4GB) SDRAM DDR3 1600
RAIDMAX RX-1200AE 1200W ATX12V v2.3 / EPS12V SLI Certified
WD Caviar Black 500GB 7200 RPM Hard Drive 3GB SATA
Creative Sound Blaster X-Fi Titanium Fatal1ty Professional
Windows 7 Premium Home Edition 64 bit
ALL games are run on a single 23" monitor set at 1920x1080

Will a pair of 660TI make a big difference?
Will one single 680 give me more for the money?
How good is my system for the upcoming games?
Thanks for your help

 

[Burgies]

Just for your reference, i moved from an MSI 560 ti Hawk to an MSI 670 Power Edition and my 3DMark11 score went from around 6000 to 9000. The 2gb memory and extra speed go a long way.