Replace/Upgrade G5 Graphics Card?

Hi. Was wondering if it is possible to replace my ATI Radeon X600 XT Graphics card in my Imac g5 w/isight. Many of the programs I have started to use are heavy on graphics and video, and i am finding this to be a little cumbersome. I recently stuck a 2 gig Ram upgrade in, but i am worried this may be all i can do to up the performance of the imac.

I suppose the question is if it is even possible to do this, would my system support another card, and are there tech services that offer this? Mac certainly does not and I am thinking it may be that changing the graphics card would entail a complete reconfiguration of the m-board, ergo -no point.