NVIDIA GeForce GTX 775M or 780M iMac, which one to choose for PS CS6?

Hi,

I'm planning to buy a new iMac 27", 16MB, 3 TB fusion drive. But for the graphics I don't know (and cannot find) what is best (would be sufficient) for Photoshop CS6 (and Lightroom-5). Would the standard GeForce GTX 775M 2GB be enough for my use or will the GeForce GTX 780M 4GB give a really better performance? It’s not the budget that’s holding me, but sometimes you get too much unused power in your Mac. I use Photoshop and Lightroom for (sometimes extensive) photo editing and not for video or 3D.

Murk