2 GB is the norm for Nvidia 670+ and there are a few 4 GB options. The 640 cards mainly come with 2 GBs with a few 1 GB options.
AMD 7950 and 7970: most cards seem to come with 3 GB. I can't find a single card on scan.co.uk that does not.
2 GB seems to be the norm these days (for desktop GPUs anyway)
With that card you might hit a wall on fill rates more than available vram, but I looked it up and every other implementation I found comes with 2GB standard, kind of like the card the mini uses typically comes with 512-1GB rather than 256. Apple is weird about ram.