what is a cross fire setup

Yep, Crossfire is basically ATIs version of SLI (not sure if you've heard of SLI either, though).
 
SLI renders parts of the screen where as Crossfire does frames.

They both require two graphics cards to be linked.
 
Once the data is sent to the cards, the next task is to divide up the actual rendering task, generating on-screen pixels from the data in the card's memory. SLI supports three methods of dividing up this task:
  • The first method, known as Split Frame Rendering (SFR), analyzes the rendered image in order to split the workload 50/50 between the two GPUs. To do this, the frame is split horizontally in varying ratios depending on geometry. For example, in a scene where the top half of the frame is mostly empty sky, the dividing line will lower, balancing geometry workload between the two GPUs. This method does not scale geometry work as well as AFR, however.
  • The second method is known as Alternate Frame Rendering (AFR), in which one GPU processes even frames, and the second processes odd frames, one after the other. When the secondary card finishes work on a frame (or part of a frame) the results are sent via the SLI bridge to the master GPU, which then outputs the completed frame. Ideally, this would result in the rendering time being cut in half, and thus performance from the video cards would double. In their advertising, NVIDIA claims up to 1.9 x the performance of one card, with the dual-card setup. However, this mode cannot be used in games that use Render To Texture functions, as neither GPU can directly access the frame buffer of the other. (There is another mode, AFR2, which can be manually chosen, but NVIDIA has not documented the difference between it and normal AFR.)
  • The third method is SLI Antialiasing. This is a standalone rendering mode that offers up to double the antialiasing performance by splitting the antialiasing workload between the two graphics cards, offering superior image quality. One GPU performs an antialiasing pattern which is slightly offset to the usual pattern, and the second GPU uses a pattern offset by an equal amount in the opposite direction. Compositing both the results gives higher image quality than is normally possible. This mode is not intended for higher framerates, and can actually lower performance, but is instead intended for games which are not GPU-bound, offering a clearer image in place of better performance. When enabled, SLI Antialiasing offers advanced antialiasing options: SLI 8X and SLI 16X. A Quad SLI system is capable of SLI 32X antialiasing.
Pretty sure most SLIs set ther self by default to SFR
 
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