Limit of dynamic memory on video cards

Hallo
I tried to gather some infos out there but I think I need to summon the league of the vvvvise men … is there any limit to the amount of memory a video card can access on the main system, any difference between integrated controllers/APUs and PCIe cards? And whatnot? What happen when the RAM is accessed from the vidoe card, will only that portion be slower or will it slow down the whole?
tx
Simone