Hi,
Does anybody know of a way to virtually combine the VRAM of two separate video cards, of different makes/models? Say I have an (onboard) VIA Unichrome and an (AGP) ATI Rage 128 Pro. Can I combine the VRAM of those and use it all for one card?

I don't believe this is something that can be done virtually since the video ram is limited to the card it self. Only way that im familiar with doing this sort of thing is with exactly the same cards and SLI

It can not be done with today's hardware. The GPU sets the video memory in protected mode. Even in SLI or Crossfire have the same limits. In these modes the drivers predicts the best way to divide the work flow to sent to each video card. What is close is framebuffer, but it is more like guess work than actually writing data to video memory. A feature around it is to use aperture memory. This type of memory shares physical system memory for more storage at a cost of performance.

The only way to actually access video memory is to design a GPU at first to include such feature. You can not joined video memory with those graphical chips, so you got what you paid for. I suggest upgrade to a better video that has the require video memory capacity. More video memory will always be preferred.