Win 98SE will actually run in 32mb RAM, but if you want to start playing games it is the game that decides how much ram is needed. Plus, of course, the speed of the processor. The box that the game came in should tell you the minimum requirements it need. So, the next question is what speed is the processor on this computer?
In the old days when I was young(er), and RAM was VERY expensive, 16M was considered a luxury. Since then software writers have assumed that we have large hard drives and lots of memory, so we now have to have at least 128M for anything to run smoothly, but as long as you don't mind waiting for your computer to write to and read from the hard drive 64M should be fine.