GTX 560 vs GTX 560 SLI

hi there

i would like to know the performance difference (in numbers if possible) between GTX 560 and GTX 560 in SLI

(non ti versions)

i bought a GTX 560 recently, and i would like to run in in SLI, playing games such as battlefield 3 , is very smooth on this card, but i wonder how much "better" it will be if i buy a second card

also please dont recommend me buying a higher model of the graphics card, as i already bought a 560 and would only like to run it in sli

(gigabyte gtx 560)

thankyou