The asus board you linked too had one 16x and one 4x. With crossfire it would only be 4x/4x. The ASRock I linked too had 16x/8x/4x, thats 28 lanes VS 20. Crossfire is 8x/8x. The Asus board only has 2 slots.
Your wrong on both accounts.
A 785 chipset only allows 16 PCIe lanes for Graphics.
The Asrock puts a Switch board for Switching 8 lanes to the second slot. If you have the switch board turned to 16 lanes for the main slot the second slot is dead. Which gives it CrossfireX support. But not Tri C/F. The third slot gets its 4 lanes from the southbridge and cant be used as a third slot for Tri C/F.
Lastly it’s down to expansion slots and the additional chipsets the board uses. Thanks to the 785G's CrossFireX support, ASRock has taken advantage of this and included a total of three PCIe x16 slots. 785G only has 16 lanes for graphics, so you either have a single x16 slot or 2 x8 slots for CrossFireX mode. (The last x16 slot is simply an x4 slot running off the Southbridge).
ASRock has been given a black mark also on its implementation of CrossFireX switching. Many other companies use digital switches to switch the PCI Express lanes, ASRock has gone and used a switch over paddle card, similar to the ones used when SLI was first released. This makes setting up CrossFireX a bit more of an effort if you already have one card installed. You have to take this video card out, switch the paddle around, reinstall that video card and then install the second video card... annoying to say the least.
http://www.tweaktown.com/reviews/29...otherboard_mainstream_785g_tested/index3.html
The Asus has a main X16 slot. It doesnt need a old switch board, it electronically switches 8 lanes to the second slot if its occupied.
2 x PCIe 2.0 x16 , support ATI CrossFireX™ technology (at one x16 or dual x8 link)
http://usa.asus.com/product.aspx?P_ID=vk9Pqxby9MjO0WHm
In CrossfireX both boards run in 8X/8X.