Windows 98se is the pinnacle of DOS based Windows operating systems. If you need support for dos games, it's the OS for you.
For just desktop work, word processing, internet, music, videos, and the like, LINUX is the OS for you. It's more stable that Windows (any version), has better online/networking, and is free. If you can't handle the learning curve, go with Windows 2000 Pro.
For gaming, Windows XP Pro is the new standard.
The *new* windows system is called Windows Vista. Beta versions are available. It follows the usual Microsoft tradition of stealing ideas from others (and patenting them), less and less efficient applications, killing off the competition with so called "improvements", and jacking the price higher and higher. There are 7 versions of it. The full version, the equivalent of XP Pro, is said to start at about $500 a pop. The smallest version will run about $100, but won't offer the benefits of things like new hardware support.