One of the coolest things about using a graphics card from MSI is their MSI Afterburner application. It is primarily meant for overclocking, but I think its monitor GUI is the best out there for GPUs. The user can pick the data he wants to be monitored (temperature, clock, memory usage and many more). These are displayed in real-time much like a heart-rate monitor. The values on the right show the current value, while hovering the mouse over any part of the graph shows the value at that time. The polling period can be adjusted and this application supports the smallest polling periods I have seen.
Tried with: MSI Afterburner 2.1.0 on a NVIDIA GTX 580