Is 256MB RAM enough for a VPS server to run some Drupal-based websites with e-shop module ?
thanks
It is enough to "run" nearly anything in the LAMP world.
It is not enough to handle more than a certain amount of load. How much depends on the app, and could be anything from 1 to tens of concurrent users.
Good enough to start with. Surely your VPS vendor will be able to upgrade your VPS if you find you need more - all you need to do then is reboot to get the extra memory.
You can probably install it, but I would recommend against using so little. Unless of course you don't have much content and/or very few views. I don't have hard numbers for you, but you may have to resort to performance testing to determine your needs.