i got a xfx 6770, and was thinking on oc'ing and decided it wouldnt really be worth it, but just me. in terms for u im not sure on how well the asus silent direct cu I fanless cooler is for cooling. Also u mean that 950/1260 max is wat u can get stable? or what u can max it out overall? like a cpu every chip is different, if ur not sure start small and see if u see an increase that makes the oc worth it.
Crossfire isn't really going to be worth it either, you won't see the performance gains you'd expect. If you're going to buy another card just get one card that's better. For another 6770 your looking at $118 after shipping ($88 after MIR). Even a HiS 7770 would be an upgrade for a single card for $130 shipped ($115 after MIR). Depends on how much you're looking to spend, but right now a single card solution is better than X Fire.