TEX itself runs with at least 1 MB memory, just for creating of a format files with initex, a half MB more does not hurt in any case.
For the Big version of TEX at least 1,5 MB should be available (for dumping accordingly more).
Running TEX with only one, resp. with two disk-drives does function, but is close to masochism. Recommended is in any case a hard-drive with ca. 5–10 MB room (according to how many fonts, macro- and format-files you want to keep on the drive at the same time).