You must have at least 550MB of memory and 850MB of hard disk space to perform a normal installation. Note that these are fairly minimal numbers. For more realistic figures, see Del 3.4, «Meeting Minimum Hardware Requirements».
Installation on systems with less memory or disk space available may be possible but is only advised for experienced users.
 Installation images that support the graphical installer require more memory than images that support only the textual installer and should not be used on systems with less than 550MB of memory. If there is a choice between booting the text-based and the graphical installer, the former should be selected on such systems.