that card is nvidia so you would be looking to use SLI (crossfire is for AMD cards).
I believe you woudl be able to use any 660 ti, but the best bet would be to get the same model asus so it would run smooth
heres another trhead where they discussed this - in case you wanted some other opinions!
http://www.tomshardware.com/answers/id-1686875/sli-brands-card.html