So basically what your saying is that it's really nothing like dual-core processing? No significant performance increase?2x 4670's are not worth crossfiring... the better option is always one more powerful card... but if really want to do crossfire i'd get 2x 4850's
Just make sure your powersupply can handle it.
It's always best to get the next best graphics card than to get 2 lower cards in crossfire/sli