Graphics Option in iMacs

I know there are several threads running but I cant find the answer to my concerns.

I am looking to buy my first iMac. I am keen on the photography front and also have a HD Video Camera which I use.

I was hoping to purchase a 20inch but realise the graphics option is the lowest. I do not play games at all and have no intention of doing so.

My question is, will the NVIDIA GeForce 9400M graphics in the base model fit my requirements or would I be better spending the extra and getting the bigger screen and better graphics card, bearing in mind this is going have to last me a few years.

Any help or guidance would be really appreciated as I have spend hours on here trying to find advice to help me decide.