If they don't, type 'help' at the DOS prompt then click on 'Boot-disk'. You may also have to edit your config. Most game manuals have a technical supplement where they detail exactly how to edit these files, including specific lines to add. Every game has its own unique requirements and what worked for Descent may not for Heretic. If you play several different games in the course of a week, it might be a good idea to create a boot disk for each one.
RAM memory is easier to deal with.
You either have enough or you don't. Some games require 4 megs of RAM as a minimum. In reality this may mean it will run fine on two and a half megs of RAM or it may need every last byte of the 4 megs. Bill Gates has spent considerably more than I'm making for this article to convince you that you need Windows 95. Windows competes with the games you're trying to run for conventional memory, RAM and hard drive space.