CrossFire X lets users to mix and match AMD cards, but why would you want to cripple the faster one with that of a slower one?
IF GTX 760 isn't much of a step up from a GTX 660, then just grab a R9-280x. The R9-280x is the fastest card for your budget.
http://www.tomshardware.com/reviews/geforce-gtx-760-review-gk104,3542.html