Which one?
I knew some years before, there was some old quad head matrox but i don't know if they support OpenGL in all the outputs.
Anyway, i don't know if it worths to buy a very expensive VGA, than a low-end pc :p
regards
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Maybe change the motherboard to have 2 pci express slots with 35 euros and buy also a new gpu (as second) with 30 euros. It does not matter if there are different model because you do not run sli. But must be the same chip brand. Nvidia or ati to have the same driver.