For RAM, more is always better..
Once you get to 2GB+ however, you should have a specific need. But if you can afford it, go for 2GB min, you'll be happy that you did.
Yes, if all you intend to run is Parallells, then 1GB will be enough, but I assume you want to use some applications as well, but in OS X and in Windows. Then you'll soon wish you had 2GB.