I use a combination of FurMark and RivaTuner
RivaTuner is a complete powerful tweaking environment, providing you everything you may need to tune NVIDIA GPU based display adapters. The widest driver-level Direct3D / OpenGL and system tuning options, flexible profiling system allowing to make custom settings on per-application basis, both driver-level and low-level hardware access modes, unique diagnostic and realtime hardware monitoring features and exclusive power user oriented tools like built-in registry editor and patch script engine make RivaTuner's feature set absolutely unmatched.
RivaTuner supports all NVIDIA display adapters starting from Riva TNT family up to the latest GeForce 7 series and widest range of NVIDIA drivers starting from the oldest Detonator 2.08 up to the newest ForceWare drivers family.
FurMark is a very intensive OpenGL benchmark that uses fur rendering algorithms to measure the performance of the graphics card. Fur rendering is especially adapted to overheat the GPU and that's why FurMark is also a perfect stability and stress test tool (also called GPU burner) for the graphics card.