Uhm... The 1Gb model is aimed towards the XFire crowd with hell-big resolutions.
So, if you play in a 30" display at max res, you *might* want the 1Gb version, but we would slap face you, lol. At that kinda res, you might want a 4870X2 or a GTX280, or a couple of those
Anyway, in a range of res below 1900x1200, the difference, like dagger said, isn't worth it right now. Maybe in the "near" future we'll see more vram demanding games :P