one 8GB or two 4GB?

[mbox_bob]

Crossfire/SLI share the video memory as both cards need to have the same frame buffer loaded, so two 4GB cards in SLI/Crossfire will still only be 4GB of VRAM (they both need the same data loaded because, in simplistic terms, each card renders every other line).
Two cards not in SLI/Crossfire don't need to have the same frame buffer, so there is 8GB of VRAM, but driven by individual cards. It does introduce other issues, such as the card which renders the game, is handled by the card which is driving the monitor the game initially starts on. If you only have 1 monitor, a second card is pointless. If you have two monitors, 1 plugged into each card, you'll find your rendering is normally only handled by one card, and if you move things between the monitors, you'll get noticeable performance hits (this can be worked around with the right tools).

Crossfire also doesn't work when apps are in Windowed mode (including windowed fullscreen), it does work when an app is in True fullscreen, which is also known as Exclusive mode, because you can only have one app in Exclusive mode. So you'd want Nvidia cards for SLI, which does work in Windowed mode - if you went down that route.

An 8GB card is just that.