How can I monitor video memory usage?
GPU load monitoring is not a built-in feature in Windows, but there are third-party tools to do the job.
GPU-Z is a graphics card information tool that supports a number of monitoring options for graphics cards such as clock speeds, fan speed, memory load, GPU load and Video Engine load.
I believe it supports both ATI (AMD) and nVidia graphics cards.
This is a screen shot of my system while playing a h.264 video.
If you use an ATI or NVIDIA card, you should check out GPU-Z.