One Gb of Ram would be adequate to contain the operating system for the mobile device but you would want a lot more memory to store data for additional apps and offline access. So it is not meaningful to ask this question stating just a single figure.
Many cheaper mobiles have limited RAM fitted as standard, to keep costs down, but allow the user to insert addition memory cards as necessary to "upgrade" the device to suit their requirements.
Of far greater importance is the speed of the processor chip and battery life, particularly if using GPS based applications such as maps while accessing the Internet. Only a few devices will allow a full days use with such applications active all the time. An additional external battery pack is a useful investment if you regularly rely on such applications.