You can use any GTX 460 1GB card to pair up in SLI. I recommend you seek out a card that has a similar factory overclock to the one you already have (815 mhz). When you pair to dissimilar cards in SLI, both cards run at the lowest clock speed of the two. So you would lose the "superoverclock", but you could overclock them yourself back up to those speeds.
Thanks, thats what I was thinking but wasnt 100% sure, thanks for the answer!