Be sure that you motherboard don“t have any bulged capacitor, i had to replace my MB for this

. it was doing blue screens of death randomly or in heavy disk load.
check also yours memory sticks, try replacing one or more ,and see if is doing the same. is a frecuent cause of BSD
Also if you want to replace your graphic card, you might need a power supply of 400 w.
I have a 6600 gt with 512 ram, 550 w ps . and runs very well despite his load times and little slowndown in ports.
but could be better a 1024 memory ( i plan put more memory)