What's funny is that Maximum PC and other enthusiast sites recommend AGAINST what you guys are saying, and I tend to agree.
You should always buy the most of what you can afford if all you care about is performance. Since the OP's monitor is so large, they would be better served by a single fast GPU solution instead of a couple of slower cards.
Originally Posted by ~Darkseeker~
but it works best with hardware that was available at the time it was release, that was the hardware it was intended to work with.
I also disagree with this statement. Hardware doesn't care what you pair it with, as long as the generations aren't too far out. They don't need to match, and it's perfectly reasonable to run a high end Core 2 Quad or Phenom II X4 with a GeForce 580 or Radeon HD 6970 card and have good performance. The i7's are all pretty damn fast, so it won't matter which graphics card (right now) you get, as it will be the limiting factor as you scale the resolutions up. The higher the resolution, the more stress you put on the GPU, NOT the CPU.
That being said, the single fastest GPU you can afford will give you the best performance *right now*, and if you do want to add another card later on to boost performance a bit until you're ready to jump, that's the ideal way to go.
Think of an upgrade path like this, for large monitors (I have one and this has served me very well)
Upgrade from prior generation card to single fastest GPU that I can afford. Buy a second one only when the first one can't do what you want, if the timeframe is still before the 2nd generation from that card is out.
Buy Radeon HD 5870. Card starts to lag in some of the newer games, buy a second 5870 because the newest generation is the 68xx/69xx family of cards (which is only one generation above what you already have). When the 7xxx generation comes out, upgrade if the single card solution is faster than the dual 5870's. Since the cards are two generations apart, the performance gap between them should be more than the standard 5-15% difference, and if you step back and look, when you buy cards every generation, you're throwing money out the window for what amounts to a pitiful generational gap. The marketing guys would have you believe that you're buying something that is revolutionary when in fact, most of the time most people don't or can't notice a difference.
The obvious exception here is that if your single 5870 is outperformed by the next generation by more than 30%, then it's OK to upgrade to the next generation, but lately, that isn't the case.
Single monitor, lower resolution cards almost never need more than one GPU to achieve high performance. Those that believe otherwise are buying into the marketing bullshit.