Is getting a 640 MB graphics card a better choice over the 768 MB?

From a gaming standpoint, would the graphics be a lot better? It's about 150 dollars more it seems like. Would it be cost effective? I want good graphics and high graphics settings, but I dont need insane graphics and outrageous prices. Also, would it be better to wait on buying the 8800, since the 9600s are coming out? Thanks much.