among other factors like case airflow, motherboard, your psu should have enough amperes on it's +12v rail(s) to power the gfx card(s) you'll be using.
for gaming at the resolution you've stated, 2gb vram per gfx card would be better. sli/cfx do not multiply/ add vram. either a gtx 570 or hd 6970 2gb or a pair of 6970s or 570s will be a very powerful combination.
the following pages are from 'the best gfx cards for the money - december 2011' article. http://www.tomshardware.com/reviews/fastest-graphics-ca... http://www.tomshardware.com/reviews/fastest-graphics-ca...
see if gtx 570, hd 6970 or 2x 6950 in cfx would fit in your budget.
For your resolution you need to get 6970 or 6950. Both have 2gb vram per card. A lot of people don't realize that todays games may not need that much ram but in 1-2 yrs it will become the norm. unfortunately the nvidia cards right now don't have enough vram on them for the same price. I wish I had a 2560 x1440 monitor because its 2xpixels compared to 1080P and once you crank up eye candy features you will need 4gb ram. I personally use dual crossfire 6970 (converted from 6950 toxic sapphire) and they run Bf3 maxed out very well.
Frankly, if you're willing to spend $700 on graphics cards then I strongly recommend waiting till 7000 series..its will have mass availability in Q1 2012. Rumours say the new cards will atleast be 10%-15% faster than GTX 580 and each card is supposed to have 3gb vram 7970 and 7950.....that is sick when you think about never having to worry about maxing out details and AA. This also means that the middle range cards will be as good as 6970 since that is 10% slower than GTX 580. Wait 2 months and get a more future proof product. These new 7000 series are rumoured to support DX 11.1 as well.
One more note...get a 2500K Instead of 2500. Even if you don't like overclocking at all you can push that to 4ghz on stock cooling and not do a thing. All you do is increase the multiplier...set the voltage to 1.3..its ultra safe...and 90% of 2500k can go to that frequency. If you're a little more brave then do 1.35 V and 4.5ghz...get a cheap $30 CM 212+ cooler. I use it and temps never go beyond 65C...All this is ultra safe with the 2500k. Its one of the best CPUs of this generation.