Many of those PCI Express cards seem to use the main system memory to power them. EXAMPLE: one might have 32mb onboard and the system memory is supposed to supply the rest to bring it to 128mb.
Please refrain from making statements that are untrue. You are propogating false information.
A PCI-E card uses no main system memory to execute commands from it's own GPU. Software written without GPU specific commands may execute commands in system memory from the main CPU.
What you might be referring to is onboard graphics (like in a laptop). Not a slot based card.
If a card says it has 256mb, it uses 256mb on the card and none from system memory.