I got 8k only after some overclocking of my GTX (490/1300). At stock frequencies I get around the 7,500 mark.
As for Crossfire (yes that name gets on my nerves too), what you have to rermember is that although it allows for two non-idenical cards to be bridged (thus being more 'user friendly' etc), I think the faster card will only runs as fast as/uses the features of the lowest common demoninator, eg a X1800 will downclock it's GPU/VRAM frequencies to that of a paired X850 XT or whatever... I stress think, as I'm not 100%.