No way man! So far I've seen only one graphics card with 512mb, some new ATI, very expensive. Not even GeForce 7800GTX Cool FX has 512mb ram. If you've got a SLI, which I doubt, because there are no VGA cards that support SLI, and besides, that would cost you completely new computer, because you have to buy a new motherboard. VGA is an old standard, all good cards have PCI-express.
So it's more like 128 or 256.