crossfire dual monitor gpu power division

Hi,

I'm currently looking at upgrading, and would like some advice about the right graphics card direction for me. I will have a dual monitor setup so I am looking at ATI over Nvidia as crossfire supports multi monitors, and would like some advice about how crossfire etc. works in this situation.

My question is If I am running 1 card, or 2 in crossfire with 2 monitors doing basic tasks etc. and say each monitor is using 10% of the max graphics power, then I power up a game in the primary monitor (say something intensive like crysis) which would use say 80% of the total cable graphics cards capable output, will the game and the primary monitor get the output required or will it only get 50% of the cards capability because the output is split evenly over the 2 monitors and the second monitor has 50% associated to it even tho it only ever uses 10%?

As a mobo like an asus p6t has 3 x16 slots, but only really 2 that can be used due to the double slot nature of most decent cards I'm wondering whether I should get 2 cards to run in crossfire, or whether I am better off with a single card for the primary monitor and a low end card to run the second monitor standalone without crossfire.

Any advice would be greatly appreciated.