you should try to find SB700 chipset kext. you can find in forum. I wrote the kext link. you will have information about chipset. but I can say you you should have proper graphic card because I couldnt run the ATI HD3200 on board GFX.

I am writing half of URL ,you can find the domain I hope

/files/160514091/Nexusmac_For_iDeneb_ATIsb700Extensions.mkext.zip

get the kext and try to add your sb 750 pci id. you should look via google. have a nice day. I hope you can do. but I think, editing the SB700 will work for you.