Its always good to search the forum http://www.tomshardware.com/forum/245454-33-crossfire-f...
"What is CrossFire ?
CrossFire is a high-performance PC Gaming Platform technology that enables multiple ATI Radeon graphics cards and a CrossFire-ready motherboard in a single computer to increase graphics quality and performance.
What is CrossFireX(Quad CrossFire)?
I found a very good definition for it in techreport.com:
CrossFireX is, quite simply, an extension of the CrossFire dual-GPU feature to three and four GPUs."
Why would you have dual over head cams compared to one on an engine?
Simple answer, more performance.
In order to use crossfire you need a motherboard with at least two PCI-E 16x length slots. It's best if both slots can operate in 16x or 8x mode, but mixing them (like using one card in a 16x slot and other in a 4x slot) is not advisable as it causes performance problems. It also depends on your CPU and settings since if your GPUs are waiting on the CPU for data they won't offer you a whole lot more performance. Some games also respond better to crossfire than others.