512 MB cards are just starting to show their age (I know, I'm running a 4870 512MB), and 1GB cards are starting to be recommended for things.
The main issue is with texture sizes. Providing the memory technology is the same, i.e. GDDR5, the 1 and 2 GB cards *should* perform more or less the same as long as the textures fit within the onboard VRAM. Once you get past that it starts to get into swapping issues which may cause some speed reductions. The 2GB cards may give you a bit more longevity, but only if the processing power of the chip can keep up. If the prices are significantly different, I would stick with the 1GB. I can live with slightly reduced texture details 2 or 3 years down the road. However, YMMV.
EDIT: Ooops. Ninja'd