Prime 95 blend test. (temps and general stability)
Intel burn test (temps and stability, more heat than prime 95)
3Dmark GPU test with some CPU stress.
Unigine heaven GPU test.
Be methodical with your overclocking and testing so you know which variable you pushed too far when instability is detected.
I would stress test with the GPU at stock settings and nail down CPU before messing with the video card.