You CAN get a normal GTX285 to work in Mac in conjunction with your GT120.
You need the GT120 still to get an output as the GTX285 doesn't output until OSX is loaded. macrumors is where to look if interested. May be worth sticking with an EVGA model though still from what I read quickly.
EVGA do a GTX285 for Mac which has a proper EFI implementation on and a £369 price as well. Is a proper Dual DVI card and works with screenoutput from boot, doesn't need to wait for OSX to load.
It is available with a 2-3 week delivery on the Apple Online Store, but isn't offered as a BTO.
Unless you need the performance you could just get a mini-dp to DVI cable to provide two DVI connections on the GT120. Would be cheaper then buying a second GT120. £68 vs £120.