I'll tell you what I'm presently using :
CPU-Z ->For monitoring the CPU configuration
GPU-Z ->For monitoring the GPU configuration/stats
FurMark ->For primary stability/artifacts tests. It's a folding application, so your card will get hot as hell.
EVGA Precision ->For GPU overclocking/in-game fps and stats
RealTemp ->For CPU temperature monitoring
Prime95 ->For CPU stressing
3DMark/PCMark Vantage/3DMark Vantage for general CPU/RAM/GPU testing
In my opinion, if you can run FurMark for 5 minutes with all settings maxed out and you don't see any artifacts/crashes/blue srcreens your GPU is 100% stable.