listen first. you have an entry level desktop config. so dnt expect ur pc 2 give u performance on the high.
i'll tell u d reason
+ ur graphics card is crashing mainly because of the power supply. 300 watts! that's shocking!
i am telling u 2 bty a new power supply unit as soon as possible coz ur pc is going too blast/burn very bad(i am nt joking it happens in a configuration like this) so buy atleast 550 w PSU bfore ur pc goes for a checkup
SAM