Using a 256mb Video Card

I have a quick question. I have been able to buy a few NOS PCI video cards with 256mb. Does this mean I need to have at least 256mb of system memory (which will be replaced with the memory on the video card) or is the video card memory an add-on to the system system memory.

For example, can I use the 256mb video card on a system which only has 64mb of main memory?

Thanks...Joe