You must have at least 780MB of memory and 920MB of hard disk space to perform a normal installation. Note that these are fairly minimal numbers. For more realistic figures, see Section 3.4, “Meeting Minimum Hardware Requirements”.
The installer normally automatically enables memory-saving tricks to be able to
run on such low-memory system, but on architectures that are less tested
it may miss doing so. It can however be enabled manually by appending the
lowmem=1 or even
parameter (see also Section 18.104.22.168, “Check available memory / low memory mode” and
Section 5.3.2, “Debian Installer Parameters”).
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 text-based installer and should not be used on systems with less than 780MB of memory. If there is a choice between booting the text-based and the graphical installer, the former should be selected on such systems.