Isis requires RISC OS 3.1 or later, an ARM250 or better, and needs at least 2 Mb of memory to really be usable. However, sheets in memory can get very large so at least 4 Mb is preferable. Disc space usage is negligable: less than 1 Mb. I believe Isis is compatible with RISC OS 4 but I have only tested on RISC OS 3.5 and am not going to upgrade.