Hi Rocknrolla
Like everyone's told you, it's your money to spend. No one is forcing you to buy a card. What we're trying to do is to tell you that for the money you end up spending, you may not end up with the best performance.
Multi-GPU setups sound nice and fancy, but to truly make the most of the them, you have to invest in a decent monitor setup, for them to stretch their legs and perform well.
at 1080p, a card like the Geforce 560 Ti or the radeon 6950 is more than sufficient to run 90% of games today at 8xAA at max details. The odd games like Metro 2033 and Witcher 2 will remain optimised for dual video cards. But for the next two years at least, these cards will serve you very well.
Think of the 580 as a Ferrari. On our roads, the Ferrari never has the space or a stretch without bumps/potholes to run at more than 250Km/h. When you can drive it at a max of 80KM/h, you are travelling the same speed as a Maruti.
Think of the road as the monitor. You need a 2560x1600 monitor to make the 580 distance itself from the single GPUs in the market, but at it's price range, a 6950x2 crossfire setup will be ahead of it as well.
Think before you spend, because this money will be lost forever and you wont get a second chance to make a better purchase once the deal is done.