2x GTX 570s or?

2 570's will roughly give you the performance of 1 590.So they're notably faster than 1 580 but they run hotter,noisier and have significantly more power consumption

You should start out by telling us what cpu and power supply you have.

If you don't have a very fast cpu, you'll just bottleneck GTX570's.

Also a cheap 1000w psu, only puts out about 600-700 real watts and would completely melt down if you tried to run those cards with it.

For the games you listed, although I have no idea what "FONV/F03" is, a single GTX570 would be plenty.

My PSU is a Kingwin Lazer 1000W, and an Intel 2500k CPU

Two 570's get 873 fps in Guru3D's Gaming Test Suite around $640
Two 900 MHz 560 Ti's get 862 fps in same suite for around $440
One 580 gets ya 616 fps in the same suite for $480
One 590 gets ya 881 fps in the same suite for $750

The VR circuitry on the 570's is not as robust as on the 560's. Ya can google the 570 issues where peeps fried the boards by exceeding manufacturer''s voltage recommendations. Granted exceeding manufacturers spec is a thing ya can't blame the manufacturers for but doing the same on the 560's has not produced the problem at similar levels.

Guru3D uses the following games in their test suite, COD-MW, Bad Company 2, Dirt 2, Far Cry 2, Metro 2033, Dawn of Discovery, Crysis Warhead. Total fps (summing fps in each game @ 1920 x 1200) for the various options in parenthesis (single card / SL or CF) are tabulated below along with their cost in dollars per frame single card - CF or SLI:

$ 205.00 560 Ti (455/792) $ 0.45 - $ 0.52
$ 220.00 560 Ti - 900 Mhz (495/862) $ 0.44 - $ 0.51
$ 320.00 570 (524/873) $ 0.61 - $ 0.73
$ 480.00 580 (616/953) $ 0.78 - $ 1.01
$ 750.00 590 (881/982) $ 0.79 - $ 1.43

In short, ya get:

1.2 % more performance with the twin 570's for 45.5 % more money compared with the twin 900Mhz 560's
28.5 % less performance with the one 580 for 9.1 % more money compared with the twin 900Mhz 560's
2.2 % more performance with the one 590 for 70.5 % more money compared with the twin 900Mhz 560's

Twin 560's - Good 750 watt PSU req'd
Twin 570's - Good 850 watt PSU req'd

Antec SG, CP*, HCP, HCG series (* requires compatible case)
Corsair HX, AX, TX V2 series
XFX Black or Core Edition
Seasonic X Series

Ok so another question. If a GPU (Nvidia or ATI) is able to run a game or program at the highest settings, what would a second GPU do to help?

SLI / CF help increase the power performance of gaming, rendering, more FPS which gives better visuals and quality here is the SLI/Crossfire FAQ
http://www.tomshardware.com/forum/245454-33-crossfire-f...
besides, 2 × HD 6970 would be better than 570s AMD 6K series have better Scaling than nVidia's

I did some research and I am almost thinking of going with a single GTX 580, then getting another one in the future if it is needed.
Would a single 580 be efficient enough for today's games? And, would 1000W be enough to power a 580 SLI?

yes it would be, and a single HD 6970 would be as well, and yea that PSU is so sufficient

those psus have a high failure rate be careful personally id replace, leds do not equate to a reliable psu 99.9% of the time

+100 the OPs psu is sketch at best (no offense) but seriously Im saying this for your benefit sell your current psu on ebay (or return it) and get a reliable unit... if you need recommendations for that I'm sure many people could supply you with links but first we need to know what your entire theoretical system would be

personally I feel that for a single 1080p display 2 gtx 560ti (either Asus dc2 or MSI TF2/3 or Gigabyte) would be the best choice imo, with a mediocre overclock (950 mhz) it is ~= to stock gtx 570 sli and considering its huge price difference is is just the best choice for people not needing the extra vram atm ie the OP

do NOT get multiple graphics cards:
http://www.tomshardware.com/reviews/radeon-geforce-stut...
(feel free to jump to the summary)

Power Supplies:
You need to factor BOTH the Wattage and the Amperage:
1) Wattage: find an online calculator to add up your parts including graphics cards
2) Amps: the AMPS on the +24V rail or rails must be at least 1.25x the value listed on the graphics card. (GTX570 needs 38Amps so get at least 48Amps for example)
*Some people spend far too much money by getting overkill on the Wattage just to be sure they have enough Amps. Follow those two rules and you can get a good PSU such as an Antec High-Amp on sale for less than $100.

New graphics cards:
I highly suggest you wait for either the AMD 7000 series (Q4 2011) or NVidia 600 series (Q2 2012).

You'll definitely need to compare benchmark values to prices as the AMD's may have an overinflated price initially with no competition from NVidia.

Graphics card?
If buying NOW I'd get either a GTX560Ti or GTX570. If possible wait for the AMD or NVidia and get one of at least the performance of a GTX560Ti (value depending).

Best answer selected by monster10888.