For your system to be capable of running games maxed out at 1080p and at a minimum of 45fps, you shouldn't need to do too much. I would suggest one very fast GPU instead of SLI for 1080p, so maybe you could go towards a GeForce GTX Titan, or an AMD Radeon HD 7970 GPU.
Either of these would max out every game out on the market at 1080p @ 45fps, without an issue. SLI and CrossFire would push out your power consumption (something like ~600-650W+ is what I'd recommend) and then your motherboard would need to be SLI- or CrossFire-capable.
I'd suggest the GTX Titan, or HD 7970, which will set you back just as much as a pair of HD 7970s.