Do you have a budget amount in mind ?
EVGA 012-P3-2066-KR GeForce GTX 560 Ti (Fermi) 448 Cores FTW 1280MB 320-bit GDDR5 PCI Express 2.0 x16 HDCP Ready SLI Support Video Card
If this card is in you budget range then this would be a great card and it was just released with the addition of 112 proccessing cores for added performance. A regular 560Ti has 336 proccessor cores and this new one has 448. If it is out of your budget range then you could consider this one;
EVGA SuperClocked 01G-P3-1563-AR GeForce GTX 560 Ti (Fermi) 1GB 256-bit GDDR5 PCI Express 2.0 x16 HDCP Ready SLI Support Video Card
$239.99 with a $20 rebate makes it $219.99http://www.newegg.com/Product/Product.aspx?Item=N82E16814130610
This chart will give you a good idea where the cards fall as far as performance level , and as you can see the 550Ti is several levels below the 560Ti and by going with two 550Ti's you will be up to the performance of the 560Ti and by going with the one 560Ti and adding another latter you can get to a performance level that is higher than a GTX 580.http://www.tomshardware.com/reviews/fastest-graphics-card-radeon-geforce,3085-7.html