Two Medium Video Cards or One High End Card

I am about to buy the video card/s for my PC. I have a ASRock Z77 Extreme6 Motherboard with
2 x PCI Express 3.0 x16 slots (PCIE2/PCIE4: single at x16 (PCIE2) / x8 (PCIE4) or dual at x8/x8 mode)

My plan is to buy a GeForce GTX 660 Ti card (cost around $280 - $300.)

My question is, since my motherboard supports dual graphics cards, could I get better performance out of multiple cards for a comparable price, or would forcing the interface to be x8 on both cards negate any advantage two cards might have?

 

You're a few steps below the Ultimate GPU there (and you haven't supplied any PSU or CPU details), go for the single card/GPU option; enjoy the gaming, not the Arms Race!