I just have a little question about updating a graphics card:does it put obstacles to play old games or games already downloaded in my pc?
Not usually. Not with modern cards anyway. You may have to find the right drivers for certain games though.
Over 10 years ago there was sometimes a problem with games that used undocumented VESA modes that weren't supported in newer video cards -- games like Angel Devoid. Nowadays you play Angel Devoid in DOSBox and for Windows games it's mostly a matter of finding the right drivers for a particular game.