Some cards work better with CF.
The 9400 GT and GTS 200 series cards seem to work very good for CrossFire.
However, I've tested alot of PC's.
Here's the results;
-AMD Athlon 64 x2 3600+ 2.1 GHz
-2GB DDR RAM
-Windows 2000
-nVidia GeForce 5200 FX
Result: 75.5 FPS average
My rig;
-AMD Phenom II X4 555 3.8 GHz (OC'ed)
-4GB DDR3 RAM 1666 MHz
-Windows 7
-nVidia GeForce GTX 460 - 768MB super clocked
Result:
Highest: 277 FPS
Lowest: 246 FPS
Average: 255 FPS
My laptop:
-AMD Turion 64 x2 2.1 GHz
-4GB DDR SO-DIMM RAM
-Windows 7
-nVidia GeForce 7150M
Result:
Highest: 60.5 FPS
Lowest: 19.4 FPS
Average: 45.5 FPS
Note that all the resolutions and settings are set to the highest available.
This is on a CLEAN install of Windows, no junk programs, hacks or viruses.
It pretty much all relies on the CPU, not very much on the GPU (although a good GPU would be good)
CF utilizes CPU usage to 55%+ or up to 100% for single-core processors.
Bottom line;
-Have a good mid-end CPU
-Have a decent low end or mid-end graphics card capable of DX9+