I was wondering what is the big difference between having 1 very good gfx card like let's say the AMD Radeon HD 6850 or having 2 smaller cards like the radeon hd 5800.
Also, which is the best for high end games like Red faction armageddon?
It would depend, largely on the type of cards you're comparing.
For example, a 6850 is designed to be at about the same level as a 5850 -- only at a different pricepoint. If you want the 5850 performance of the 6xxx series, you move to a 69xx card.
If you're comparing 5850's in Crossfire to a single 6850, the crossfire cards will of course come out better.
I've no idea how crossfired 5830's would compare to a single 6850, though.
mostly used for more monitors, but overall, it ain't that much better. two smaller cards are slightly better then one giant card though, but the difference ain't that much.
But you gotta remember that you can actually place them (not just slots, but also space) and your motherboard can handle it (I forgot that).