I would try to reset the cmos first. Turn off the computer, unplug the computer, open the case and find the small battery on the motherboard. Make sure you ground yourself by touching a piece of metal from the case before taking the battery out. Leave it out for about 20 minuets. Push the power button a couple times also (I have heard there can be some stored power left in the caps?) Put the battery back and start the computer. I doubt this will do anything, but it is a quick thing to try.
If I had to guess, I would say it is your power supply. I say that because I see you added a fairly power hungry graphics card plus more ram. You may be reaching the limits of your power supply. How big and what kind of power supply is it? Should be listed on the side of it.