First of all, this doesn't belong in the CPU forum.
Second of all, could you be a little clearer? "is it common fact in all PCs?" What do you mean? What are you trying to ask?
As for the upgrade, hell, that whole PC from top to bottom is crap by today's standards. You'd probably be best aiming at upgrading the graphics card if you want to game on it, but that's still going to be pretty sad. And as for your next question, which graphics card to use, take that to the graphics card forum please.