Most likely, It could also be the RAM of the GPU, but I doubt it. The best way to see if its your RAM is to download and try
Memtest 86. Check the site for all the instructions (its a free DL).
You can also pop out one of the sticks of RAM and try to reboot. This might give you an idea of which stick of ram it is.
Run memtest and see what it gives you, then post back with your results. Good luck.