Ok, I'd let Memtest run overnight. Sometimes it takes a few passes to find bad spots.
I'd recommend Coretemp (http://www.alcpu.com/CoreTemp/)
for monitoring CPU temperatures, and Speedfan (http://www.almico.com/sfdownload.php)
for monitoring GPU temps, but there's more. I'd use Furmark (http://www.ozone3d.net/benchmarks/fur/)
to get GPU temps up quickly. When you run Furmark, you need to enable "Stability test", or something like that, it will have it run until you tell it to stop, and has a temp monitor built in.
And get that dust out of there!