The latest NVIDIA web drivers have removed the hardware restriction and now install on all systems instead of just a cMP.
http://www.nvidia.com/download/driverResults.aspx/88655
People are reporting some significant performance improvements with these drivers, so it might be worth installing them if you have an NVIDIA GPU in your iMac. As always, if you run into any issues, you can simply use the NVIDIA preference pane to switch back to the stock Apple-provided drivers.
http://www.nvidia.com/download/driverResults.aspx/88655
BETA support is for iMac 14,2 / 14,3 (2013), iMac 13,1 / 13,2 (2012) and MacBook Pro 11,3 (2013), MacBook Pro 10,1 (2012), and MacBook Pro 9,1 (2012) users.
People are reporting some significant performance improvements with these drivers, so it might be worth installing them if you have an NVIDIA GPU in your iMac. As always, if you run into any issues, you can simply use the NVIDIA preference pane to switch back to the stock Apple-provided drivers.