Sorry ... I got confused. It was another, similar thread where Crossfire was explained.
Crossfire is basically a way to use two ATI video cards together to increase the performance. It won't double your performance, but it gives a good boost.
Basically, think on to when you want to upgrade. A new graphics card is, say, $100, gives a 50% boost. Another 4670 for Crossfire is $50, gives a 50% boost. What are you going to choose?
That's just an example though, and is Crossfire turns out not to be feasible because the cards turns out to be expensive in a few months or a year, you can always buy another card anyway.
It's just another upgrade option, it doesn't have to be used.
Fallout 3 should run fine, Crysis on medium/some high settings, Unreal Tournament 3 no problem, WoW I'm not sure, CoD 4 no problem, Alone in the Dark should be fine. Mass Effect 2 I'm not sure, haven't seen any info on performance yet.