to demonstrate a example
this picture here if used as a background (choosen randomly from google )
http://images.bit-tech.net/news_images/2008/10/gigabyte-shows-bit-tech-its-final-x58s/Gigabyte-X58-UD5P.jpg
resolution is 2061 × 1734 and is just using 1.18 mb and that's vram of your dedicated graphics card and there unless you would have a graphics card with only 4 mb vram that amount is nothing to worry about ,
but as you got if i am not mistaken a PowerMac G5 (the amount of 8GB ram is suggesting that) you got at least 64mb vram ,so just ~ 1/64th of your vram is used to display that picture on the screen .
so your 8GB RAM are not effected at all unless you would have a shared memory graphic's card which you definitely dont have in a PowerMac