_,,,^..^,,,_ said:

So, it seems that AviSynth and/or VirtualDub DO USE GPU power too, or the old GPU simply slowed down the CPU speed?

AviSynth does not natively use the GPU. However there are a handful of plug-in filters that do, e.g. DGDecNV runs on the GPU of Nvidia graphics cards that support CUDA video decoding, and those source filters that use the DirectShow framework (DGAVCDecDI, DirectShowSource and DSS2) may use hardware accelerated decoding (dependent on filter merit, I imagine). And there is also FFT3dGPU which is a filter used for denoising/sharpening.

So if your script uses any of those it may benefit from an upgraded graphics card.