Chipset in GT120, GT130, HD4850 iMacs

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I was wondering what chipset is used in the iMacs with discrete graphics? I'm thinking it would use the same 9400M chipset as the lower models, but I haven't found a definitive answer. I'm guessing the IGP is there, but is disabled until Snow Leopard along with the obligatory $10 enablement fee to satisfy the accountants. Having the 9400M would certainly be useful as another OpenCL target to complement the discrete GPU, even without SLI.