SLI'ing two 560 900MHz cards fits ya budget and its outdoes two 6970's at 1920 x 1200 ... also faster than a 580. At 2560 x 1600, Id get the 6970's tho.
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:
$210.00 560 Ti (455/792) $0.46 - $0.53 (Scaling = 174%)
$315.00 6970 (526/
825) $0.60 - $0.76 (Scaling = 157%)
$220.00 560 Ti - 900 Mhz (495/
862) $0.44 - $0.51 (Scaling = 174%)
$500.00 580 (616/953) $0.81 - $1.05 (Scaling = 155%)
Total $1632
Case - $120 - Antec 902 V3
http://www.newegg.com/Product/Product.aspx?Item=N82E16811129097
PSU - $110 - XFX 750 WBlack Edition PSU
http://www.newegg.com/Product/Product.aspx?Item=N82E16817207003
MoBo - $385 - ASUS P8P67 Pro
http://www.newegg.com/Product/ComboDealDetails.aspx?ItemList=Combo.642515
CPU - inc - Intel Core i5-2500K included w/ above
Cooler - $40 - Scythe SCMG 2100
http://www.newegg.com/Product/Product.aspx?Item=N82E16835185142
TIM - $5 - Shin Etsu
http://www.newegg.com/Product/Product.aspx?Item=N82E16835150080
RAM - $75 - (2 x 4GB) Corsair CAS 9
http://www.newegg.com/Product/Product.aspx?Item=N82E16820145324
GFX - $220 - EVGA GTX 560 Ti 900 Mhz
http://www.newegg.com/Product/Product.aspx?Item=N82E16814130651
GFX - $220 - Same
HD - $65 - Spinpoint F3 1TB 7200 rpm
http://www.newegg.com/Product/Product.aspx?Item=N82E16822152185
SSD - $300 - OCZ Vertex 3 2.5" 120GB
http://www.newegg.com/Product/Product.aspx?Item=N82E16820227706
DVD Writer - $22 - Asus 24X DRW-24B3L w/ LS
http://www.newegg.com/Product/Product.aspx?Item=N82E16827135221
OS - $100 - Win 7-64 Home Pre
http://www.newegg.com/Product/Product.aspx?Item=N82E16832116754
Upgrade to the Z68 MoBo for $ 40 more ($425 total)
$200
http://www.newegg.com/Product/Product.aspx?Item=N82E16813131729
$225 http://www.newegg.com/Product/Product.aspx?Item=N82E16819115072