I know it isn't *recommended* but I have also had good success with cards on eBay that have hacked firmwares and run at 4gb video ram. Work great on the old cheese grater macs.
there can definitely be driver issues with these things
some gtx680 hack dont return correct pci bus info
other use experimental drivers
not something id recommend
but newer nvidia cards can possibly be supported by their drivers - in unhacked versions
http://netkas.org/?p=1356