i would still go with dual 560ti or dual 570s in sli over a 590/6990. sli/crossfire isnt hard to set up (basically put the cards in as you would one card, than put the sli/crossfire bridge on, which is very simple to do, than make sure the drivers are set to use SLI/CF, again, simple to do).
the OP doesnt have build experience
so might not be so simple for him
really though in general going for a higher end single card solution is
better than going for a dual medium level solution
remember further down the line he could SLI/CF the HD6990/GTX590
A year from now he could have a 2600k with dual GTX 590s/HD 6990s
I sure wouldnt complain about that