first of all u must have ati 3200 chipset motherboard for this...it started as i poped in two x1800xt 512mb cards out of curiosity into my at8 32x abit mobo, only to find out that the crossfire tab was there, but grayed out 
...i have read that x1800gto and x1900gt would work but ati confirmed that xt are not supported...after reading over on futuremark forum that someone was able to get x1800xl 256mb to run in xfire without any mods, that got me to thinking if i can mod the xt cards to work in pair...luckily there is a bios out by msi that is x1800xl 512mb so i flashed that to xt (i guess thats a first ever lol) card, and the crossfire checkbox was now available ...luckily the ram voltages in that bios are the same as the xt...i used rabit 2.1 to set the core voltage to 1.3 and to copy xt latencies, and now both cards work happily on 1.3v 600/700 in crossfire...3dmark jumped from 5000 to around 8500, and there is no image corruption or skipping of frames so the chipset is working flawlesly...now i dont have overdrive cause the xl bios does not support it, but i can always tune up he cards to 1.48v 700/800 if i need to with atitool...like for that damn call of juarez game ...nyways just putting info out
...i have read that x1800gto and x1900gt would work but ati confirmed that xt are not supported...after reading over on futuremark forum that someone was able to get x1800xl 256mb to run in xfire without any mods, that got me to thinking if i can mod the xt cards to work in pair...luckily there is a bios out by msi that is x1800xl 512mb so i flashed that to xt (i guess thats a first ever lol) card, and the crossfire checkbox was now available ...luckily the ram voltages in that bios are the same as the xt...i used rabit 2.1 to set the core voltage to 1.3 and to copy xt latencies, and now both cards work happily on 1.3v 600/700 in crossfire...3dmark jumped from 5000 to around 8500, and there is no image corruption or skipping of frames so the chipset is working flawlesly...now i dont have overdrive cause the xl bios does not support it, but i can always tune up he cards to 1.48v 700/800 if i need to with atitool...like for that damn call of juarez game ...nyways just putting info out 
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