1st both of them are from different company
2nd both of them are for different usage
3rd both of them have different specification
there is nothing call "when do i need Nvidia, when do i need ATi", which card u select is depending on ur usage, and as well as which specification of the card, and in the end u still need a graphic processor, if not u have nothing to see on ur monitor

HD2600 has lower specification compare to 8800GS in term in its processing power and RAM
if u require higher graphic processing power or heavy usage on graphical thing then go for 8800GS, if u didnt use alot or just normal use and some minor gaming HD2600 will do, i only using the HD2400, can do most of my thing out of it