Minimum requirements include:
8 Mbytes of memory (of course, more is better..)
100 Mbytes of free disk space
~40 mb for executables,
~40 mb for source code (which you can ignore if you merely install the Linux binaries)
~? for data (the veritable bottomless pit can be filled with data, if you so choose)
GRASS runs on Linux kernel versions as old as 1.2.13 (see more information in the appendices for various specific binaries).
GRASS will run in text mode. However, for displays of data, you will need X. If you are still running a version of X, it will probably work with GRASS.
If you find any other hardware/OS requirements that should be mentioned, please let me know!