OK, ya haven't given us much info .....
< $200, I'd recommend the 6870
> $200, I'd recommend the 560 Ti (factory OC'd model as they same price and come w/ bigger coolers and more robust VRM's.
That's based on the following:
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:
$ 170.00 6870 (434/701) $ 0.39 - $ 0.49
$ 220.00 6950 (479/751) $ 0.46 - $ 0.59
$ 240.00 6950 Frozr OC (484/759) $ 0.50 - $ 0.63
$ 205.00 560 Ti (455/792) $ 0.45 - $ 0.52
$ 320.00 6970 (526/825) $ 0.61 - $ 0.78
$ 215.00 560 Ti - 900 Mhz (495/862) $ 0.43 - $ 0.50
$ 340.00 570 (524/873) $ 0.65 - $ 0.78
$ 500.00 580 (616/953) $ 0.81 - $ 1.05
With twin 580's giving ya just 10% more fps than the twin 900 Mhz 560 Ti's, I can't see spending more than twice as much for the 10% .... but if ya got the dough, I can't see a reason to talk ya outta it either
To do the SLI thing, you'll want at least a 750 watt PSU, better to have the 850 watter if OC'ing the GPU further as well as the CPU....I have seen those 560's hit 30+% overclocks
http://www.pureoverclock.com/review.php?id=1201&page=17
Of course a roomy, cool case and decent SLI MoBo must e part of the equation.
Here's a $1875 build w/ "everything" hardware wise except monitor and OS that I did for a home entertainment PC for a neighbor.
Case - $ 160 - Antec 1200 V3
http://www.newegg.com/Product/Product.aspx?Item=N82E16811129100
Case Fans - $ 17 - Antec Blue 120 mm
http://www.newegg.com/Product/Product.aspx?Item=N82E16835209011
PSU - $ 115 - Antec CP-850
http://www.newegg.com/Product/Product.aspx?Item=N82E16817371024
MoBo - $ 210 - ASUS P8Z68-V Pro /GEN3
http://www.newegg.com/Product/Product.aspx?Item=N82E16813131790
CPU - $ 220 - Intel Core i5-2500K
http://www.newegg.com/Product/Product.aspx?Item=N82E16819115072
Cooler - $ 50 - Hyper 612 PWM
http://www.newegg.com/Product/Product.aspx?Item=N82E16835103103
RAM - $ 46 - (2 x 4GB) Corsair Vengeance
http://www.newegg.com/Product/Product.aspx?Item=N82E16820233186
GFX - $ 230 - Asus GTX 560 (900MHz)
http://www.newegg.com/Product/Product.aspx?Item=N82E16814121424
GFX - $ 230 - Same
HD - $ 150 - Spinpoint F3 1TB
http://www.newegg.com/Product/Product.aspx?Item=N82E16822152185
SSD - $ 210 - Mushkin Chronos Deluxe 120 GB
http://www.newegg.com/Product/Product.aspx?Item=N82E16820226225
DVD Writer - $ 59 - Asus 24X DRW-24B3L w/ LS
http://www.newegg.com/Product/Product.aspx?Item=N82E16827135247
Keyboard - $ 89 - Logitech G510
http://www.newegg.com/Product/Product.aspx?Item=N82E16823126100
Mouse - $ 50 - Logitech G500
http://www.newegg.com/Product/Product.aspx?Item=N82E16826104318